Transducer to convert optical energy to electrical energy

ABSTRACT

An optical transducer, optoelectronic device, and semiconductor are disclosed. An illustrative optical transducer is disclosed to include a plurality of p-n stacks, where each p-n stack comprises at least a p-layer and an n-layer, and formed therein a built-in photovoltage between the p-layer and the n-layer. The p-layers and n-layers are disclosed to have substantially the same n-type material in substantially the same composition such that each p-n stack in the plurality of p-n stacks has a substantially similar built-in photovoltage. The optical transducer is further disclosed to include a plurality of connecting layers, each connecting layer in the plurality of connecting layers being sandwiched between two adjacent p-n stacks for electrically connecting the two adjacent p-n stacks. The p-n stacks in the plurality of p-n stacks may be arranged such that the built-in photovoltage of each p-n stack additively contributes to an overall electric potential of the transducer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-Part of U.S. patent applicationSer. No. 15/609,771, filed on May 31, 2017, which is aContinuation-in-Part of U.S. patent application Ser. No. 14/565,141,filed on Dec. 9, 2014, which claims the benefit of priority of U.S.Provisional Patent Application No. 61/913,675 filed Dec. 9, 2013; eachof which are hereby incorporated herein by reference.

FIELD

The present disclosure relates to devices that convert optical energyinto electrical energy.

BACKGROUND

In the technological area of photovoltaics, there exist applicationsthat require the efficient conversion of a narrowband optical inputsignal into an electrical output signal, for example by producing adirect current (DC) electrical output. In some of these applications,the narrowband optical input signal is provided by a laser or a lightemitting diode.

Prior art devices for converting the optical input signal into theelectrical output signal include photovoltaic single junction devices,which typically produce a low-voltage electrical output, which can betoo constraining for designers. The prior art devices also includephotovoltaic multi-junction devices that can have multiple photovoltaicjunctions arranged in a same plane and electrically connected to eachother in series. Such photovoltaic multi-junction devices can producehigher voltage output signal; however, they require that themulti-junctions have the same dimensions, be arranged symmetrically, andthat the impinging narrowband input optical signal be shaped anddirected towards the multi-junctions in a way that uniformly illuminateseach junction. Further, such photovoltaic multi-junction devices canrequire complicated fabrication and assembly techniques including, forexample, the need for a plurality of deep trenches with high aspectratio etches in the various semiconductor layers, air bridge metalconnectors, multi-level contacts and connections, thick dopedsemiconductor layers to minimize the sheet resistivity (for example forback contacts or top contacts), etc.

Therefore, improvements in transducer that convert an input opticalsignal into an output electrical signal are desirable.

SUMMARY

In a first aspect, the present disclosure provides an transducerconfigured to convert optical energy to electrical energy. Thetransducer is disclosed to include a plurality of p-n stacks, where eachp-n stack includes at least a p-layer and an n-layer, and formed thereina built-in photovoltage between the p-layer and the n-layer. In someembodiments, all of the p-layers include substantially the same p-typematerial in the same composition and all the n-layers includesubstantially the same n-type material in the same composition such thateach p-n stack has a substantially similar built-in photovoltage. Thetransducer is further disclosed to include a plurality of connectinglayers, each connecting layer being sandwiched between two adjacent p-nstacks for electrically connecting the two adjacent p-n stacks. In someembodiments, each p-n stack is arranged such that the built-inphotovoltage of each p-n stack adds up to form an overall electricpotential of the transducer.

Other aspects and features of the present disclosure will becomeapparent to those ordinarily skilled in the art upon review of thefollowing description of specific embodiments in conjunction with theaccompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure will now be described, by way ofexample only, with reference to the attached Figures.

FIG. 1 shows a prior art p-n junction.

FIG. 2 shows an embodiment of a transducer in accordance with thepresent disclosure.

FIG. 3 shows a flat band energy diagram of a homogeneous p-n diode.

FIG. 4 shows four segments of the p-n diode of FIG. 3.

FIG. 5 shows the four segments of FIG. 3 electrically connected to eachother with connecting elements.

FIG. 6 shows an embodiment of a transducer of the present disclosurethat has connecting elements that form tunnel diodes.

FIG. 7 shows an embodiment of a connecting element of the presentdisclosure.

FIG. 8 shows another embodiment of a connecting element of the presentdisclosure.

FIG. 9 is a graph of current as a function of voltage of a p-n diodehaving no connecting elements.

FIG. 10 is a graph of current as a function of voltage for an embodimentof a transducer in accordance with the present disclosure.

FIG. 11 shows expected performance variations for an embodiment of atransducer according to the present disclosure when the wavelength ofthe optical input is varied from an optimum value.

FIG. 12 shows expected performance variations of a transducer accordingto the present disclosure when the thicknesses of the base layersegments are varied from optimum design value.

FIG. 13 shows the conversion efficiency as a function of optical inputpower for an embodiment of a transducer in accordance with the presentdisclosure.

FIG. 14 shows the open circuit voltage as a function of optical inputpower for an embodiment of a transducer in accordance with the presentdisclosure.

FIG. 15 shows a top view of a transducer of the present disclosure thatcomprises grid lines separated from each other by a distance of 325microns.

FIG. 16 shows a top view of transducer of the present disclosure thatcomprises separated from each other by a distance of 425 microns.

FIG. 17 shows a top view of a transducer of the present disclosure thatis free of gridlines but that comprises a transparent conductive film.

FIG. 18 shows the averaged short circuit current (Isc) for thetransducers of FIGS. 15, 16, and 17.

FIG. 19 shows fill factor data for the transducers of FIGS. 15, 16, and17.

FIG. 20 shows a plot of detected power as a function of measuretransducer voltage for an embodiment of a transducer of the presentdisclosure.

FIG. 21 shows an embodiment of a transducer and data receiver unit inaccordance with the present disclosure.

FIG. 22 shows an embodiment of a power meter in accordance with thepresent disclosure.

FIG. 23 shows a bandgap energy diagram for a semiconductor material.

FIG. 24 shows the input power dependence of the short circuit currentmeasured from single GaAs p/n junctions of different thicknesses.

FIG. 25A shows plots of optical to electrical conversion efficiencies asa function of optical power for a series of p/n junctions.

FIG. 25B shows plots of optical to electrical conversion efficiencies asa function of optical power intensity for a series of p/n junctions.

FIG. 26 shows plots of responsivity as a function of applied voltage fora same optical transducer illuminated at various optical powers.

FIG. 27 shows the relative performance of a same photo transducer as afunction of the wavelength of the optical input for two differentillumination powers.

FIG. 28 shows the relative performance of a same photo transducer as afunction of optical input power at two different illuminationwavelengths.

FIG. 29 shows a cutaway perspective view of a prior art optical fibercable assembly.

FIG. 30 shows a cross-sectional view of a prior art flange mountconnector.

FIG. 31 shows a cross-sectional view of an embodiment of a connector inaccordance with the present disclosure.

FIG. 32 shows a top front perspective view of an optical fiber cableassembly facing a board mount connector in accordance with the presentdisclosure.

FIG. 33 shows a top back perspective view of the connector and theoptical fiber optic cable of FIG. 32.

FIG. 34 shows mated the optical fiber cable assembly and the board mountconnector of FIGS. 32 and 33.

FIG. 35 shows an embodiment of a system in accordance with the presentdisclosure.

FIG. 36 shows a flowchart of method according to an embodiment of thepresent disclosure.

FIG. 37 shows another prior art p-n junction.

FIG. 38 shows a connecting layer according to an embodiment of thepresent disclosure.

FIG. 39 shows details of a connecting layer according to an embodimentof the present disclosure.

FIG. 40 shows a series of layer pairs used to construct a connectinglayer according to an embodiment of the present disclosure.

FIG. 41 shows effective bandgap estimation as a function of aluminumcomposition according to an embodiment of the present disclosure.

FIG. 42 is a cross-sectional view of a prior art tunnel junction.

FIG. 43 is a cross-sectional view of a tunnel junction according to anembodiment of the present disclosure.

FIG. 44 is a detailed cross-sectional view of a tunnel junctionaccording to an embodiment of the present disclosure.

DETAILED DESCRIPTION

Generally, the present disclosure provides a Harmonic PhotovoltageUp-converter for High-Efficiency Photon to Direct-Current (DC)Phototransducer Power Conversion Applications. A transducer converts oneform of energy into another form of energy, for example optical energyinto electrical energy. The phototransducer of the present disclosureconverts an optical input into and an electrical output preferably witha high conversion efficiency based on a direct current/voltage output.The optical input preferably includes a beam of light, for example alaser light beam. The beam of light is typically propagated from thesource to the phototransducer. The propagation of the beam of light canbe done with a collimated beam thru free-space, such as air, othergases, or vacuum. Alternatively the propagation of the beam of light canbe done through a solid medium such as an optical fiber, a waveguide, ora light guiding medium which can also include liquid a medium, which canall be considered part of the light source. For example thephototransducer of the present disclosure can be placed at the receivingend of a light beam propagated thru an optical fiber to deliver powerfrom a distant source to a remote equipment which will use the voltageor current converted by the phototransducer. For clarity, a lightsource, preferably in the form of a beam, provides an optical input forthe phototransducer of the present disclosure.

State-of-the-art PV devices are most often based on a single p-njunction such as shown in FIG. 1, which includes an emitter 16 and abase 18 made of semiconductors doped with dopants of opposite polarity(p or n). The base 18 and the emitter 16 form a p-n junction 104, andtogether have a thickness (t) 120 of semiconductor material that absorbsan optical input 100 which is impinging the front surface 102 of the PVdevice. The performance of the single junction PV device can be improvedby using a passivating window 14, doped with the same doping type as theemitter 16 and disposed between the front surface 102 and the emitter16. A contact layer 12 and/or a metal layer 10 can be used to extractthe current and voltage generated by the single junction PV device. Thecontact layer 12 and/or the metal layer 10 can be patterned usingstandard photolithography techniques or deposited such that only afraction of the front surface 102 is covered in order to efficiently letthe optical input reach the absorbing thickness 120 of the semiconductoremitter 16 and base 18.

Optically transparent but electrically conducting layers can also beused in conjunction with or in substitution to the contact or metallayers 12, 10. The layers such as the base 18 and the emitter 16 aretypically grown on a substrate 24 which can serve as a mechanicalsupport and which can define the lattice constant of the semiconductorcrystal grown on the substrate 24. A buffer layer 22 can be disposedbetween the substrate 24 and the base 18 to adjust the crystal qualityand/or for other fabrication, optical, electrical, or crystal growthpurposes. For example the buffer 22 can be used to provide electricalconductivity (lateral sheet conductivity or vertical conductivity toextract the current) or to change the optical properties of the singlejunction PV device. The performance of a single junction PV device canbe further improved by using a back surface field (BSF) layer 20, dopedwith the same doping type as the base 18 and disposed between thesubstrate 24 and the base 18. The optical input 100 is absorbed withinthe thickness 120 of the emitter and base, and the minorityphoto-carriers in the base and in the emitter are swept across the p-njunction 104 developing a photo-current and photo-voltage across anexternal circuit which can be connected to the top metal 10 and thesubstrate 24.

For a PV device, the optimum conversion performance, often referred toas the conversion efficiency, defined as the ratio of the optical powerimpinging from on the PV device to the electrical power output from thePV device, is obtained when the PV device is operated near or at themaximum power point of the current-voltage curve (I-V curve) of the PVdevice. The maximum power point of the I-V curve corresponds to thepoint at which the product of the output voltage (Vmax) and the outputcurrent (Imax) has reached a maximum. That point defines the fill-factorcoefficient (also known has the filling factor coefficient)FF=(Vmax*Imax)/(Voc*Isc), where Voc is the open circuit voltage of thePV device and Isc is the short current value of the PV device. It issometimes also convenient to define the ‘voltage FF’ (FFv), and the‘current FF’ (FFi) as FFv=Vmax/Voc and FFi=Imax/Isc, such thatFF=FFv*FFi. The PV device can be operated under differentcurrent-voltage settings by changing the impedance of the load to whichit is electrically connected. For example an external load connected tothe PV device with the optimal impedance, also known as the optimumload, optimum impedance, or optimum circuit resistance, will drive thephototransducer to operate near its maximum power point.

The optical input signal has a light wavelength (λ or lambda), a lightfrequency (ν), a light energy (or photon energy) E=hν or equivalentlyE=hc/λ, where h is Planck's constant and c is the speed of light. Theoptical input signal can be monochromatic in which case the photonscomprising the optical input all have the same wavelength and frequency.The optical input signal can comprise a band of wavelength or colors, inwhich case the photons comprising the optical input all have awavelength and frequency around a mean value. The width of the band ofwavelength can be referred to as the bandwidth of the optical inputsignal. For example the optical input signal can be broadband ornarrowband. The optical input signal can have more than one band ofwavelengths, for example multiple bands each band being centered at amean frequency. In the case of multiband optical input signals, the PVdevice can be optimized for one of the band of wavelengths. For thedescription below, when the light input has a band of frequencies orwavelengths, then the mean frequency or wavelength of that band will betaken as the frequency or the wavelength of the optical input signal. Inthe case for which the optical input has a band of frequencies orwavelengths, then the PV device can be optimized for a narrow bandoptical input, for example, for a bandwidth of less than 500 nm, or abandwidth of less than 100 nm, or a bandwidth of less than 20 nm.

The optical input signal can be said to have an equivalent lightphotovoltage: hν/e, where e is the charge of the electron. For exampleif the optical input has a wavelength of λ=830 nm, the light energy isE=1240 eV*nm/830 nm=1.494 eV. Then the light photovoltage of the opticalinput signal is 1.494V. The transducer of the present disclosure iscapable of converting the optical input signal to voltage that isgreater than that of the optical input photovoltage. For example, insome embodiments, the transducer of the present disclosure is capable ofconverting and optical input signal having a photovoltage of about 1.5volts into an electrical output voltage comprised between 2V and 20V, orbetween 2V and 12V, or between 2V and 8V. The output voltage of thetransducer can be the maximum power voltage point of the transducer(often labelled Vmpp, or Vmax). Alternatively, the photovoltage outputof the phototransducer can be any voltage that the device can producebetween 0V and the open circuit voltage Voc. The photocurrent output ofthe transducer can be the maximum power current point of the transducer(often labelled Impp, or Imax). Alternatively, the photocurrent outputof the transducer can be any current that the device can produce between0 A and the short-circuit current Isc.

An embodiment a transducer of the present disclosure is shown in FIG. 2.Similarly to a state-of-the-art single junction PV device of the priorart shown in FIG. 1, the phototransducer device of the presentdisclosure in FIG. 2 includes a base 18, and an emitter 16 made ofsemiconductors doped with dopants of opposite polarity (p or n). Thebase and the emitter form a p-n junction 104, and together have athickness (t) 120 of semiconductor material absorbing the optical inputsignal 100 which is impinging the front surface 102 of the transducer.The present embodiment includes a passivating window 14, doped with thesame doping type as the emitter and disposed between the front surface102 and the emitter 16. A contact layer 12 and/or a metal layer 10 canbe used to extract the current and voltage generated by the transducerdevice. The contact layer and/or the metal layers can be patterned usingstandard photolithography techniques or deposited such that only afraction of the front surface 102 is covered in order to efficiently letthe optical input reach the absorbing thickness 120 of the semiconductoremitter 16 and base 18. Optically transparent but electricallyconducting layers can also be used in conjunction with or insubstitution to the contact or metal layers 12, 10. The layers such asthe base 18 and the emitter 16 are typically grown on a substrate 24which serves as a mechanical support and which defines the latticeconstant of the semiconductor crystal. A buffer layer 22 can be disposedbetween the substrate 24 and the base 18 to adjust the crystal qualityand or for other fabrication, optical, electrical, or crystal growthpurposes. For example the buffer 22 is preferably used to provideelectrical conductivity (lateral sheet conductivity or verticalconductivity to extract the current) or to change the optical propertiesof the PV device. A back surface field (BSF) layer 20 is preferablyused, doped with the same doping type as the base 18 and disposedbetween the substrate 24 and the base 18. The BSF and window layers arepreferably used to reflect the minority carriers back toward the p/njunction. For example if the base is p-type, the minority carriers inthe base are the photo-electrons, then the emitter would be n-type andthe minority carriers in the emitter would be the photo-holes. In thatcase the window would preferably have a band discontinuity in thevalence band to reflect the minority holes in the emitter back towardthe p/n junction 104, and the BSF would preferably have a banddiscontinuity in the conduction band to reflect the minority electronsin the base back toward the p/n junction 104. The optical input 100 isabsorbed within the thickness 120 of the emitter and base, and theminority photo-carriers in the base and in the emitter are swept acrossthe p/n junction 104 developing a photo-current and photo-voltage acrossan external circuit which can be connected to the top metal 10 and thesubstrate 24.

Furthermore, the embodiment of the present disclosure incorporatesconnecting elements 106 (c1), 108 (c2), 110 (c3), and 112 (c4). In someembodiments, the connecting elements may be referred to as connectinglayers in that one or more layers of material are used to fabricate theconnecting elements. The connecting elements are positioned at specificdistances away from the surface of the emitter 104 closest to the frontsurface 102. For example as illustrated in FIG. 2, connecting element c1106 is positioned at a distance d1 from the emitter and windowinterface, similarly connecting element c2 108 is positioned at adistance d2 from the emitter and window interface, connecting element c3110 is positioned at a distance d3 from the emitter and windowinterface, connecting element c4 112 is positioned at a distance d4 fromthe emitter and window interface. FIG. 2 exemplifies an embodiment with4 connecting elements; other embodiments can include more than 4connecting elements or less than 4 connecting elements without departingfrom the scope of the present disclosure.

FIGS. 3, 4, 5, and 6 make use of semiconductor band diagrams toillustrate how the electrical output voltage of the transducer can bemade higher than the input light photo voltage (hν/e) by usingconnecting elements in accordance with the present disclosure.

FIG. 3 shows a flat band energy diagram of a homogeneous p-n diode 999in equilibrium with no biasing and no illumination. The lines 1000 and1002 represent respectively, the conduction band and the valence band.The bandgap energy 1004 is the energy difference between the conductionband 1000 and valence band 1002. The length 1006 of the lines representsthe thickness of the semiconductor layers that make up the p-n diode999. The dotted line 1008 represents the Fermi level (Ef) through thep-n diode 999. The position of the Fermi level 1008 with respect to theconduction and valence band is determined by the doping concentration ofthe semiconductor material present in the p-n diode 999 and the type ofdonors, n-type or p-type. The p-n diode 999 has an n-doped region 1001and a p-doped region 1003. The n-doped region 1001 is the emitterregion, the p-doped region is the base region. The Fermi level 1008 isbetween the valence band 1002 and the conduction band 1000. The dopingcharacteristics of the semiconductor material or materials that make upthe p-n diode can be chosen to have the Fermi level 1008 to be withinKT˜0.025 eV or less from the conduction band in n-type material andsimilarly within 0.025 eV or less from the valence band in p-typematerial in order to thermally activate as many donors as possible. “KT”is the product of the Boltzman's constant K and the temperature T inkelvins. This is shown in the flat band energy diagram of FIG. 3, wherethe Fermi level 1008 shifts its position with respect to the conductionband 1000 and with respect to the valence band 1002 throughout the p-ndiode 999 going from the n-doped region 1001 to the p-doped region 1003.The shift or variation in the Fermi level 1008 from the n-doped region1001 to the p-doped region 1003 can be referred to as a built-inelectric potential or as an electric potential 1010, which can bedenoted V_(bi). The built-in electric potential 1010 defines the turn-onvoltage of the p-n diode. The built-in potential 1010 also defines thevoltage produced by the p-n diode 999 upon being illuminated with lighthaving an energy at least equal to that of the bandgap energy 1004.Examples of built-in potentials include, for Si, about 0.7 eV; for GaAs,1 eV; for InP, 1.3 eV; for Ge, 0.3 eV, for InGaAs, 0.4 eV; for InGaP,1.4 eV; for AlInP, 2.0 eV; for AlInAs, 1.2 eV; for InAs, less than 0.1eV. Heterogeneous semiconductors that comprise two or more differentsemiconductors have built-in potential that are commensurate with theircomposition.

The p-n diode 999 can absorb light in accordance with an absorptioncoefficient, which is a function of the materials that make the p-ndiode. The total amount of light that can be absorbed by the p-n diodeis a function of the absorption coefficient and of the thickness 1006 ofthe p-n diode.

FIG. 4 shows four segments 1012, 1014, 1016, and 1018 of the p-n diode999 of FIG. 3. The four segments of FIG. 4 are shown spaced-apart forclarity reasons; however, it is to be understood that the sum of thethicknesses of the segments is equal to the thickness 1006 shown at FIG.3.

FIG. 5 shows the four segments 1012, 1014, 1016, and 1018 electricallyconnected to each other with connecting elements 1020, 1022, and 1024.Each connecting element adjoins (i.e., is in contact with) two of thefour segments. Each connecting element is configured to introduce anelectric potential between the segments to which each connecting elementis connected (electrically connected). The successive built-in electricpotentials V_(bi), V′_(bi), V″_(bi), and V′″_(bi) add up to introduce atotal built-in electric potential equal toV_(total)=V_(bi)+V′_(bi)+V″_(bi)+V′″_(bi). When all the built-inelectric potentials are equal to V_(bi), V_(total)=4*V_(bi).

FIG. 6 shows an embodiment of a transducer of the present disclosurewhere the connecting elements comprise highly doped semiconductors thatform tunnel diodes 1021, 1023, and 1025. One skilled in the art wouldknow that a pair of highly doped layers comprising a highly doped n-typelayer adjacent to a highly doped p-type layer can form a tunnel diode,which, when combined with other semiconductor layers can be referred toas a tunnel diode unit. The tunnel diode unit can be used to passcurrent from one section to another and or change the polarity of thesemiconductor layer sequence in the heterostructure. The n and the pdoping level can be 10¹⁸ cm⁻³ or higher up to 5×10²¹ cm⁻³.

Each of FIGS. 3, 4, 5, and 6 has a pair of vertical scales, one of whichcorresponds to energy and the other to voltage. In each of FIGS. 3, 4,5, and 6, the p-n diode receives light at the emitter side, which is atthe n-doped region 1001 side.

FIGS. 3, 4, 5, and 6 show an equilibrium situation where no externalvoltage is applied and no light is present. In practice, transducers(phototransducers) operate in a forward bias mode with impinging light.A person skilled in the art will understand that the concepts of quasiFermi levels for both electrons and holes can be introduced to betterfine tune and optimize these practical situations. However, the basicprinciples leading to the output voltage of a transducer being higherthan the input light photo-voltage (hν/e) are well known and illustratedin these Figures, which are described above.

FIG. 7 shows an example of a connecting element or connecting layer 300that can be used as the connecting element c1 106, c2, 108, c3, 110,and/or c4 112 shown in FIG. 2; the connecting element 300 of FIG. 7 canalso be used as the connecting element 1020, 1022, and/or 1024 shown atFIG. 5. The following describes the connecting element 300 in relationto base segments 1014, 1016, and 1018 of FIG. 5. The connecting element300 comprises a first layer 302 closest to the front surface of thephototransducer (closest to the n-doped region 1001 shown at FIG. 3) andis p-doped, as are the base elements 1014, 1016, and 1018 of FIG. 4. Thebandgap Eg1 of layer 302 is greater than the bandgap energy of the baseelements 1014, 1016, and 1018, and, as such, the layer 302 istransparent to the optical input signal, which has a photon energy valueclose to that of the bandgap energy 1004 (see FIG. 3).

The connecting element 300 further comprises a second layer 304 that iselectrically connected to first layer 302. The second layer 304 is alsop-doped; however, the concentration of dopants in the second layer 304is higher than in the first layer 302. The bandgap Eg2 of second layer304 is greater than the bandgap energy of the base elements 1014, 1016,and 1018 and, as such, the second layer 304 is transparent to theoptical input signal.

The connecting element 300 further comprises a third layer 306 that iselectrically connected to the second layer 304. The third layer 306 isn-doped with a high concentration of dopants, the concentration ofdopants in the third layer 306 is similar to that of the second layer304. The bandgap Eg3 of layer 306 is greater than the bandgap energy ofthe base elements 1014, 1016, and 1018 and, as such, the second layer304 is transparent to the optical input signal.

The connecting element 300 further comprises a fourth layer 308electrically connected to the third layer 306 and is n-doped butpreferably but at a dopant concentration that is lower than that of thethird layer 306. The bandgap Eg4 of layer 308 is greater than thebandgap energy of the base elements 1014, 1016, and 1018 and, as such,the second layer 304 is transparent to the optical input signal.

The connecting element 300 further comprises a fifth layer 310electrically connected to the fourth layer 308. The fifth layer 310 isn-doped at a similar or lower dopant concentration than the fourth layer308. The bandgap Eg5 of layer 310 is the same or greater than thebandgap of the base elements 1014, 1016, and 1018.

Numerous other embodiments of the connecting elements 300 can bedesigned by incorporating a number of semiconductor layers either feweror higher than the layers illustrated and described in FIG. 7, andwithout departing from the scope of the present disclosure. As anexample, FIG. 8 shows an embodiment where, instead of having the secondand third layers 304 and 306, there is a conductor layer 312. Otherspecific examples are described further below. N-type and p-type dopingof the various layers of transducer can be effected through any suitabletechnique using any suitable type of dopants. For an example, for InP,GaAs, and related compounds, n-type doping can effected by, for example,Si, Ge, Te, S, Sn atoms, and p-type doping can be effected by, forexample, Zn, C, Cd, Mg, Si, Ge, Cr, and Be atoms.

The number of connecting elements present in a transducer in accordancewith the present disclosure will help determine the output voltage ofthe transducer: for example, when the output voltage of the p-n diode999 of FIG. 3 element has a value X, then the output voltage of atransducer of the present disclosure having one connecting element willbe approximately 2× or, the output voltage of a transducer of thepresent disclosure designed with two connecting elements will beapproximately 3×, or the output voltage of a transducer of the presentdisclosure designed with three connecting elements will be approximately4×, or, the photovoltage of the a transducer of the present disclosuredesigned, as illustrated in FIG. 2, with 4 connecting element c1 106, c2108, c3 110, and c4 112 will be approximately 5×. The exact value of thevoltage (photovoltage) generated by each base segment may vary slightly.The variations can be caused for example because of the differentthickness of the individual base segments, because of a differentbuilt-in electric fields resulting from different thicknesses or dopingprofiles, because of a different doping concentration in each basesegment, because of the photon reabsorption from one base segment toanother, or because of other growth or fabrication parameters such asthe growth temperature used during the growth of the various segments,or slight variations in alloy composition during the growth. Forexample, a thinner base segment will typically generate a higherphotovoltage, due to the higher probability of extraction of thephoto-carriers. For example, the above, or other, intrinsic or extrinsiceffects can result in different effective ideality factors, orn-factors, in the various base segments. Importantly, the transducer ofthe present disclosure can readily benefit from higher photo-voltagesgenerated from such thinner base segments. This is in contrast to thecase for which a prior art phototransducer is constructed without themultiple base segments and connecting elements disclosed herein. In thelatter case, a prior art phototransducer that would be designed with ap-n junction having a thinner base would also typically exhibit a higherphotovoltage than the equivalent p-n junction having a thicker base, butit would have lower overall performance because the thinner base willnot be capable of absorbing all of the photons from the optical input.In the present disclosure, the input light that is not absorbed in theupper base segments is absorbed in the lower base segments, such thatsubstantially all the photons of the optical input are absorbed.However, because each individual base segment is thinner than the basesegment thickness that would be necessary to absorb all the incominglight, each base segment can generate a slightly higher photovoltagethan it would if it had been thicker.

As will be understood by the skilled artisan, the number of connectingelements required may be commensurate with the desired output voltage ofa transducer in accordance with the present disclosure. For example, ifX is the output voltage of a p-n diode such as shown at referencenumeral 999 in FIG. 3, which does not have any collecting element, theY=(n+1)X will be the output voltage of a transducer according to thepresent disclosure having n connecting element(s), where n is an integernumber having any suitable value. For example n can be between 1 and 20,or between 1 and 10, or between 1 and 5. That is the output voltage of atransducer according to the present invention is a multiple of theoutput voltage of a similar transducer that does not incorporate anyconnecting element. Correspondingly, it could be said that thephotovoltage of the phototransducer according to the present disclosureas an output photovoltage which is a harmonic value of the photovoltageof a similar PV device not incorporating the connecting elements, asillustrated in FIG. 2 and described herein. For clarity, the harmonicvalue or the multiplication factor of the output voltage of the presentdisclosure is determined by the number of connecting elements formed inthe transducer.

With reference to FIGS. 1 and 2, to achieve the desired optoelectronicproperties of the phototransducer of the present disclosure, theconnecting elements in the embodiment of FIG. 2 divide the thickness tof the base 18 of the transducer into multiple base segments. Forexample the base segment s1 130, the base segment s2 132, the basesegment s3 134, the base segment s4 136, and the base segment s5 138, asillustrated in FIG. 2. The thicknesses of the various base segments (s1,s2, s3, s4, s5) can be the same or they can be different by adjustingthe position of the connecting elements. That is the position d1 of c1,the position d2 of c2, the position d3 of c3, and the position d4 of c4can be adjusted to change the thicknesses of the various base segments.The values of d1, d2, d3, d4 can be chosen such that the thickness of s5138 is greater than the thickness of s4 136, and the thickness of s4 136is greater than the thickness of s3 134, and the thickness of s3 134 isgreater than the thickness of s2 132, the thickness of s2 132 is greaterthan the thickness of s1 130.

The value of d1, d2, d3, d4 can be chosen such that each base segment(s1, s2, s3, s4, s5) absorbs substantially the same fraction of thephotons from the optical input 100. For example s1 absorbs 20% of thephotons from the optical input, s2 absorbs 20% of the photons from theoptical input, s3 absorbs 20% of the photons from the optical input, s4absorbs 20% of the photons from the optical input, and s5 absorbs 20% ofthe photons from the optical input. To obtain an ideal phototransducer,all the base segments taken together absorb substantially all thephotons from the optical input 100. But for manufacturing or costconsiderations, and depending on the application for thephototransducer, it may be desirable that all the base segments togetherabsorb less than all the photons from the optical input 100. Forexample, the various base segments s1, s2, s3, s4, and s5 can absorbeach 19.8% of the optical input 100. It will also be clear for oneskilled in the art that the thickness t of the base 18 and theabsorption coefficient of the semiconductor material used to constructthe base will be important factors in determining the fraction of theoptical input signal absorbed by each base segment and in total by thegroup of base segments all together.

Therefore, for most direct bandgap III-V semiconductors, the thickness tof the base 18 will not exceed 5 microns and will, in some embodiments,be between 3 microns and 4 microns. For example, neglecting thereflection at the surface of the transducer, which is a validapproximation for transducer having an antireflection coating at thefront surface 102, the light intensity transmitted at a depth z from thesurface of the semiconductor is given by I(z)=I_(o) exp(−αz). I_(o) isthe input intensity (such as the initial intensity of the optical inputsignal 100 in FIG. 2) and α is the semiconductor absorption coefficient,which is a function of the wavelength dependent density of states of thematerial (i.e., α is wavelength dependent). For a sufficiently thicksemiconductor material, only light having a wavelength longer than thesemiconductor bandgap equivalent wavelength (or with an energy less thanthe bandgap energy) will transmit through the semiconductor layer sincethe density of states drops, as does α, for photon energies less thanthe bandgap energy. For direct bandgap semiconductors, at wavelengthsshorter than the bandgap wavelength, α is in the 10⁴ cm⁻¹ to 10⁵ cm⁻¹range and each impinging photon can create a pair of photocarriers; thatis, an electron and a hole.

The semiconductor in the base layer 18 can be substantiallylattice-matched or pseudomorphic to the substrate 24. Epitaxial layersthat do not have strain-induced defects are often called pseudomorphiclayers. Pseudomorphic heterostructures can contain strained layers butonly to the extent that elastic deformations are able to accommodatethat strain such that no defects are generated by excess stresses orstrains in the device. That is the lattice constant of the semiconductorused for the base layer 18 or the base segments 130, 132, 134, 136, and138 is preferably substantially the same as the lattice constant of thesubstrate 24. Lattice-matched layers help assure good crystal quality,low defect densities, long minority carrier lifetimes, low parasiticcurrents, low dopant diffusion, low alloy diffusion, low shunting, andtherefore higher transducer performance. Alternatively, the base layer18 can be metamorphic and therefore have a lattice-mismatched to thesubstrate 24. For metamorphic layers, the buffer 22 can be used toadjust the lattice parameter of the semiconductor layers from thelattice parameter of the substrate 24 to the desired lattice constantvalue for the base 18. For metamorphic layers, the buffer 22 can begraded in composition or comprise various layers with different latticeconstants to accommodate the stresses or the strains due to thelattice-mismatched.

Examples of lattice-matched or pseudomorphic semiconductors that can beused for the base layer 18 or the base segments 130, 132, 134, 136, and138 for embodiments comprising a GaAs substrate 24 include: binary GaAs,AlAs, or ZnSe; or ternary Al_(x)Ga_((1-x))As, In_(x)Ga_((1-x))P,Al_(x)Ga_((1-x))P; or quaternary InGaAsP, GaInNAs. As will be understoodby a worker skilled in the art, other alloys can also be used such as:group IV semiconductors: Ge, SiGe; other III-V alloys of AlGaInNPAsSb;other II-VI alloys of ZnCdMnMgOSSeTe, or combinations of the above.Other embodiments can incorporate nanostructures to enhance the opticalproperties, electronic properties, and/or material properties. Forexample the base 18 could comprise layers of semiconductor quantumwells, quantum wires, or quantum dots.

Examples of lattice-matched or pseudomorphic semiconductors that can beused for the base layer 18 or the base segments 130, 132, 134, 136, and138 for embodiments comprising a Ge substrate 24 include:latticed-matched Ge, In_(x)Ga_((1-x))As, InAlGaAs, In_(x)Ga_((1-x)) P,Al_(x)Ga_((1-x))P; or binary GaAs, AlAs, or ZnSe; or ternaryAl_(x)Ga_((1-x))As; or quaternary InGaAsP, GaInNAs. As will beunderstood by a worker skilled in the art, other alloys can also be usedsuch as: group IV semiconductors: Ge, SiGe; other III-V alloys ofAlGaInNPAsSb; other II-VI alloys of ZnCdMnMgOSSeTe. Other embodimentscan incorporate nanostructures to enhance the optical properties,electronic properties, and/or material properties. For example, the base18 could include layers of semiconductor quantum wells, quantum wires,or quantum dots.

Examples of lattice-matched or pseudomorphic semiconductors that can beused for the base layer 18 or the base segments 130, 132, 134, 136, and138 for embodiments comprising a InP substrate 24 include:latticed-matched InP, In_(x)Ga_((1-x))As, In_(x)Al_((1-x))As,In_(x)Ga_((1-x))P_(y)As_((1-y)), GaAs_(y)Sb_((1-y)), Zn_(x)Cd_((1-x))Se,ZnSe_(y)Te_((1-y)); or GaInNAs. As will be understood by a workerskilled in the art, other alloys can also be used alone or combinedtogether comprising: Ge, SiGe; other III-V alloys of AlGaInNPAsSb; otherII-VI alloys of ZnCdMnMgOSSeTe. Other embodiments can incorporatenanostructures to enhance the optical properties, electronic properties,and/or material properties. For example the base 18 can comprise layersof semiconductor quantum wells, quantum wires, or quantum dots.

Examples of lattice-matched or pseudomorphic semiconductors that can beused for the base layer 18 or the base segments 130, 132, 134, 136, and138 for embodiments comprising a Si substrate 24 include:latticed-matched Si, GaP, AlP, Al_(x)Ga_((1-x))P, ZnS, GaPN, AlPN,Al_(x)Ga_((1-x))PN. As will be understood by a worker skilled in theart, other alloys can also be used alone or combined togethercomprising: Ge, SiGe; other III-V alloys of AlGaInNPAsSb; other II-VIalloys of ZnCdMnMgOSSeTe. Other embodiments can incorporatenanostructures to enhance the optical properties, electronic properties,and/or material properties. For example the base 18 can comprise layersof semiconductor quantum wells, quantum wires, or quantum dots.

Examples of lattice-matched or pseudomorphic semiconductors that can beused for the base layer 18 or the base segments 130, 132, 134, 136, and138 for embodiments comprising a GaN, a SiC, or a Sapphire substrate 24include: latticed-matched GaN, or AlInN. As will be understood by aworker skilled in the art, other alloys can also be used alone orcombined together comprising: ZnO, or other III-V alloys ofAlGaInNPAsSb; other II-VI alloys of ZnCdMnMgOSSeTe. Other embodimentscan incorporate nanostructures to enhance the optical properties,electronic properties, and/or material properties. For example the base18 could comprise layers of semiconductor quantum wells, quantum wires,or quantum dots.

Examples of lattice-matched or pseudomorphic semiconductors that can beused for the base layer 18 or the base segments 130, 132, 134, 136, and138 for embodiments comprising a InAs or GaSb substrate 24 include:latticed-matched InAs, GaSb, CdSe, AlSb, InPSb, ZnTe, CdSTe, CdSeTe,MnSeTe. As will be understood by a worker skilled in the art, otheralloys can also be used alone or combined together comprising: otherIII-V alloys of AlGaInNPAsSb; other II-VI alloys of ZnCdMnMgOSSeTe.Other embodiments can incorporate nanostructures to enhance the opticalproperties, electronic properties, and/or material properties. Forexample the base 18 could comprise layers of semiconductor quantumwells, quantum wires, or quantum dots.

Examples of lattice-matched or pseudomorphic semiconductors that can beused for the base layer 18 or the base segments 130, 132, 134, 136, and138 for embodiments comprising a CdTe or InSb substrate 24 include:latticed-matched CdTe or InSb. As will be understood by a worker skilledin the art, other alloys can also be used alone or combined togethercomprising: other III-V alloys of AlGaInNPAsSb; other II-VI alloys ofZnCdMnMgOSSeTe. Other embodiments can incorporate nanostructures toenhance the optical properties, electronic properties, and/or materialproperties. For example the base 18 could comprise layers ofsemiconductor quantum wells, quantum wires, or quantum dots.

FIG. 2 exemplifies an embodiment of a transducer in accordance with thepresent disclosure where GaAs or Ge is used for the substrate 24. Thesubstrate is p-type to have the same conductivity as the base 18 whichis also preferably p type. The base layer 18 can be GaAs, or can includea fraction x of Al using an Al_(x)Ga_((1-x))As to optimize thephototransducer's performance according to the wavelength of the opticalinput 100. The thickness of the base layer is between 1 and 10 microns,or between 2 and 5 microns, or between 3 and 4 microns. A back surfacefield 20 made of p-type GaInP provides a good minority carrier reflectorfor the minority electron photo-generated in the base 18 by the opticalinput 100. The thickness of the back surface field is between 10 nm and2 microns, or between 20 nm and 500 nm, or between 80 and 120 nm. Abuffer layer 22 is optional but can be used to insure that the epilayersurface and/or the growth conditions are optimized before the growth ofthe optically active layers atop the substrate and buffer layers. Thebuffer layer 22 can be p-type GaAs or p-type GaInP. The thickness of thebuffer is between 10 nm and 10 microns, or between 100 nm and 2000 nm,or between 200 and 500 nm. The emitter 16 can be n-type GaAs, or caninclude a fraction x of Al using an n-type Al_(x)Ga_((1-x))As tooptimize the phototransducer's performance according to the wavelengthof the optical input 100. Alternatively an n-type GaInP can also be usedfor the emitter layer 16. The thickness of the emitter layer 16 can bebetween 10 nm and 1000 nm, or between 20 nm and 200 nm, or between 80and 120 nm.

A passivating window 14 (which can also be called a passivation window,or simply a window layer) made of n-type GaInP or AlInP provides a goodminority carrier reflector for the minority holes photo-generated in theemitter 16 by the optical input 100. The window layer can comprise anumber of layers with different values of doping and bandgap energies tooptimize the optoelectronic properties of the phototransducer. Thewindow layer can be adjacent to, and in electrical contact with theemitter layer and the contact layer. The window layer can also playother functions in the device to improve the crystal quality, tooptimize the photon absorption and photo-carrier extraction, and/or totransition the growth process from one section of layers to another. Thedoping in the window layers can also be increased to minimize resistivelosses due to the current flowing from and into the emitter of the topjunction 16 to the top n-type ohmic metal contact 10. The thickness ofthe window can be between 10 nm and 5 microns, or between 20 nm and 2000nm, or between 30 and 1000 nm. The window layer can be transparent tothe optical input 100. Alternatively the window composition could alsobe Al_(x)Ga_((1-x))As with an Al composition x such that the band gap ofthe window is greater than the energy of the photons from the opticalinput 100 and also greater than the bandgap of the emitter 14. Thewindow can also be divided into 2 sections: a first section adjacent tothe emitter having a lower bandgap than the second section adjacent tothe front surface 102 and/or the contact layer 12. For example, thefirst section of the window can be made of GaInP and the second sectioncan be made of AlInP.

The ratio of the thickness of the first section over the total thicknessof the window can be between 10% and 90%. The window layer can also be astop-etch in the fabrication process of the phototransducer. For examplea wet-etch solution can be chosen to stop on a layer containing acertain concentration of Al, or P, and the design of the layercomposition can therefore take into considerations such elements whichwould facilitate the fabrication processes.

A contact layer 12 can be made of n-type GaAs. The doping level in thecontact layer is made high enough to insure a good ohmic contact with alow resistivity with the metal layer 10 deposited atop the contact layer12. The doping level of the contact layer 12 can be comprised between5×10¹⁷ cm⁻³ and 5×10²⁰ cm⁻³, or between 10¹⁸ cm⁻³ and 10¹⁹ cm⁻³. Thethickness of the contact layer 12 can be comprised between 100 nm and5000 nm, or between 20 nm and 1000 nm, or between 30 and 500 nm.

The embodiment exemplified in FIG. 2 and described herein gives anexample of a n on p configuration, but a worker skilled in the art wouldrecognize that a p on n configuration would give equivalent features andis also within the scope of the present disclosure. For a p on nconfiguration the doping types are reverse, for example the base 18would be doped n-type and the emitter would be doped p type, etc.

The doping level for the base layer 18 or the base segments 130, 132,134, 136, and 138 can be between 5×10¹⁵ cm⁻³ and 10¹⁹ cm⁻³, or between5×10¹⁶ cm⁻³ and 8E18 cm⁻³, or between 5×10¹⁷ cm⁻³ and 5×10¹⁸ cm⁻³. Thedoping level for the emitter layer 16 can be comprised between 10¹⁶ cm⁻³and 5×10¹⁹ cm⁻³, or between 10¹⁷ cm⁻³ and 10¹⁹ cm⁻³, or between 5×10¹⁷cm⁻³ and 5×10¹⁸ cm⁻³. The doping level for the window layer 14 can becomprised between 10¹⁷ cm⁻³ and 10²⁰ cm⁻³, or between 5×10¹⁷ cm⁻³ and5×10¹⁹ cm⁻³, or between 10¹⁸ cm⁻³ and 10¹⁹ cm⁻³. The doping level forthe back surface field layer 20 can be comprised between 10¹⁷ cm⁻³ and10²⁰ cm⁻³, or between 5×10¹⁷ cm⁻³ and 5×10¹⁹ cm⁻³, or between 10¹⁸ cm⁻³and 10¹⁹ cm⁻³.

As mentioned above, the connecting elements separate the base 18, intomultiple base segments, each of can absorb substantially the samefraction of the optical input, and the connecting elements electricallyconnect adjacent base segments to each other. The connecting elementscan be made of metal layers, or transparent conducting oxides layers, ortunnel junction layers, or doped semiconductor layers. The connectingelements may include a semiconductor layer that is doped oppositely tohow the semiconductor base layers are doped. That is, the connectingelements may include a semiconductor layer that is n-doped when thesemiconductor base layers are p-doped and, the connecting elements mayinclude a semiconductor layer that is p-doped when the semiconductorbase layers are n-doped. The various base segments can be mechanicallystacked and disposed between the connecting elements or grown on top ofeach other while being intercalated between the connecting elements. Tofacilitate the transmission of the optical input from one base segmentto the others, and also to facilitate the fabrication process and thequality of the semiconductor materials in the various base segments, itcan be advantageous to grow all the base segments and the connectingelements using a continuous epitaxial process. Using different bandgapenergy semiconductor materials for the connecting elements asillustrated in FIG. 7 can also be beneficial. It may also be desirableto optimize the doping level and doping type of the varioussemiconductor layers comprised in the connecting element in order tofacilitate the current flow, to optimize the photo-carrier (electron andhole) extraction in presence of the connecting elements, and to allowthe photovoltages and photocurrents of the various base segments to beconnected. The current flow can comprise the flow of majority carriersor minority carriers within the valence bands or the conduction bands ofthe transducer, the tunneling of carriers from a conduction band to avalence band or from a valence band to a conduction band, the conversionof minority carriers into majority carriers using p-n junctions, thedefect, trap, or mini-band assisted carrier transport, or anycombination with other suitable current flows.

For an embodiment in which the base 18 is GaAs, for example p-type GaAsas shown in FIG. 2: layer 302 can be GaInP, AlGaInP, AlAs or AlGaAs witha composition chosen to give a band gap greater than the energy of thephotons of the optical input, and the p-type doping level for layer 302can be comprised between 10¹⁷ cm⁻³ and 10²⁰ cm⁻³, or between 5×10¹⁷ cm⁻³and 5×10¹⁹ cm⁻³, or between 1×10¹⁸ cm⁻³ and 10¹⁹ cm⁻³. The thickness ofthe layer 302 can be between 10 nm and 2 microns, or between 20 nm and500 nm, or between 80 nm and 120 nm. Layer 304 can be made of GaInP,AlGaAs, AlAs or AlInP with a composition chosen to give a bandgap energygreater than the energy of the photons of the optical input, and thep-type doping level for layer 304 can be comprised between 10¹⁸ cm⁻³ and10²¹ cm⁻³, or between 5×10¹⁸ cm⁻³ and 5×10²⁰ cm⁻³, or between 7×10¹⁸cm⁻³ and 5×10²⁰ cm⁻³. The thickness of layer 304 can be comprisedbetween 5 nm and 2 microns, or between 10 nm and 500 nm, or between 20and 200 nm. Layer 306 can be made of GaInP, AlGaAs, AlAs or AlInP with acomposition chosen to give a bandgap energy greater than the energy ofthe photons of the optical input, and the n-type doping level for layer306 can be comprised between 10¹⁸ cm⁻³ and 10²¹ cm⁻³, or between 5×10¹⁸cm⁻³ and 5×10²⁰ cm⁻³, or between 7×10¹⁸ cm⁻³ and 5×10²⁰ cm⁻³. Thethickness of layer 306 can be comprised between 5 nm and 2 microns, orbetween 10 nm and 500 nm, or between 20 and 200 nm. Layer 308 can bemade of GaInP, AlGaAs, with a composition chosen to give a band gap Eg4greater than the energy of the photons of the optical input, and then-type doping level for layer 308 can be comprised between 10¹⁷ cm⁻³ and10²⁰ cm⁻³, preferably between 5×10¹⁷ cm⁻³ and 5×10¹⁹ cm⁻³, or between10¹⁸ cm⁻³ and 10¹⁹ cm⁻³. The thickness of layer 308 can be comprisedbetween 10 nm and 2 microns, or between 20 nm and 500 nm, or between 80nm and 120 nm. Layer 310 can comprise GaAs, GaInP, or AlGaAs, with acomposition chosen to give a bandgap energy Eg5 equal or greater thanthe energy of the photons of the optical input, and the n-type dopinglevel for layer 310 can be comprised between 10¹⁶ cm⁻³ and 5×10¹⁹ cm⁻³,or between 10¹⁷ cm⁻³ and 10¹⁹ cm⁻³, or between 5×10¹⁷ cm⁻³ and 5×10¹⁸cm⁻³. The thickness of layer 310 can be comprised between 10 nm and 1000nm, or between 20 nm and 200 nm, or between 80 nm and 120 nm.

For the case for which the base 18 is InP, or InGaAsP with a latticeconstant similar to the one of InP, or InGaAs with a lattice constantsimilar to the one of InP, for example, p-type InP, p-type InGaAs, orp-type InGaAsP, the layer 302 can be made of InP, AlInAs, GalnAsP, orAlGaInP, with a composition chosen to give a bandgap Eg1 greater thanthe energy of the photons of the optical input, and the p-type dopinglevel for layer 302 is between 10¹⁷ cm⁻³ and 10²⁰ cm⁻³, or between5×10¹⁷ cm⁻³ and 5×10¹⁹ cm⁻³, or between 10¹⁸ cm⁻³ and 10¹⁹ cm⁻³. Thethickness of layer 302 can be comprised between 10 nm and 2 microns, orbetween 20 nm and 500 nm, or between 80 nm and 120 nm. Layer 304 can bemade of InP, AlInAs, GalnAsP, or AlGaInP, with a composition chosen togive a bandgap energy Eg2 greater than the energy of the photons of theoptical input, and the p-type doping level for layer 304 can becomprised between 10¹⁸ cm⁻³ and 10²¹ cm⁻³, or between 5×10¹⁸ cm⁻³ and5×10²⁰ cm⁻³, or between 7×10¹⁸ cm⁻³ and 5×10²⁰ cm⁻³. The thickness ofthe layer 304 can be comprised between 5 nm and 2 microns, or between 10nm and 500 nm, or between 20 nm and 200 nm. Layer 306 can be made ofInP, AlInAs, GalnAsP, or AlGaInP, with a composition chosen to give abandgap energy Eg3 greater than the energy of the photons of the opticalinput, and the n-type doping level for layer 306 can be comprisedbetween 10¹⁸ cm⁻³ and 10²¹ cm⁻³, or between 5×10¹⁸ cm⁻³ and 5×10²⁰ cm⁻³,or between 7×10¹⁸ cm⁻³ and 5×10²⁰ cm⁻³. The thickness of layer 306 canbe comprised between 5 nm and 2 microns, or between 10 nm and 500 nm, orbetween 20 nm and 200 nm. Layer 308 can be made of InP, AlInAs, GalnAsP,or AlGaInP, with a composition chosen to give a bandgap energy Eg4greater than the energy of the photons of the optical input, and then-type doping level for layer 308 can be comprised between 10¹⁷ cm⁻³ and10²⁰ cm⁻³, or between 5×10¹⁷ cm⁻³ and 5×10¹⁹ cm⁻³, or between 10¹⁸ cm⁻³and 10¹⁹ cm⁻³. The thickness of layer 308 can be comprised between 10 nmand 2 microns, or between 20 nm and 500 nm, or between 80 nm and 120 nm.Layer 310 can be made of InGaAs, InP, AlInAs, GalnAsP, or AlGaInP, witha composition chosen to give a bandgap energy Eg5 equal or greater thanthe energy of the photons of the optical input, and the n-type dopinglevel for layer 310 can be comprised between 10¹⁶ cm⁻³ and 5×10¹⁹ cm⁻³,or between 10¹⁷ cm⁻³ and 10¹⁹ cm⁻³, or between 5×10¹⁷ cm⁻³ and 5×10¹⁸cm⁻³. The thickness of layer 310 can be comprised between 10 nm and 1000nm, or between 20 nm and 200 nm, or between 80 nm and 120 nm.

The connecting elements 300 as illustrated in FIG. 7 are disposed withinthe base 18 as illustrated in FIG. 2 at specific distances from theemitter. As mentioned above, these distances depend on the wavelength ofthe optical input 100 and on the absorption coefficient of the basematerial 18. A worker skilled in the art will recognize that there aremany possible combinations of optical wavelengths and base materialswhich can be used to satisfy different applications. To exemplify anembodiment of the disclosure, the description below details examplepositions of the connecting elements for a phototransducer of thepresent disclosure using an optical input 100 with a wavelength of 830nm. The semiconductor material for the base layer is chosen to have abandgap energy smaller than the energy of the photons of the opticalinput 100. The bandgap energy can be between 500 meV and 0 meV smallerthan the energy of the photons of the optical input 100. In otherembodiments, the bandgap energy can be between 200 meV and 10 meVsmaller than the energy of the photons of the optical input 100. Yet inother embodiments, it might be desirable to design the phototransducerwith the bandgap energy to be slightly larger than the energy of thephotons of the optical input 100 in the case for which the semiconductormaterial has residual absorption below the bandgap energy. For awavelength of 830 nm, the photon energy is 1.494 eV, therefore the basesemiconductor material can be chosen to be preferably GaAs, orAl_(x)Ga_((1-x))As with x comprised between 0% and 5%. If a p-type GaAsbase is chosen as illustrated in FIG. 2 with a thickness 120 of t=4microns, then the absorption coefficient α for GaAs at the optical inputwavelength of 830 nm is approximately 1.2×10⁴ cm⁻¹. Therefore based onthe formula for the absorption I(z)=I_(o) exp(−αz) it is straightforwardto calculate that the value for the position of the connecting elementsof d1=192 nm, d2=438 nm, d3=784 nm, and d4=1365 nm will divide the baseinto 5 segments, s1, s2, s3, s4, and s5, each absorbing 19.8% of theoptical input light.

A semiconductor material with a bandgap energy larger than the opticalinput can be selected for the emitter 16, for example Al_(x)Ga_((1-x))Aswith x between 10% and 35% or InGaP. Alternatively, GaAs can also beselected for the emitter 16, but then the value of the thickness of theGaAs emitter layer 16 has to be subtracted from the above values of d1to d5 because the optical input 100 will also be absorbed in the emitterlayer 16. Similarly, for the layer 310 with Eg5 of the connecting layersc1, c2, c3, and c4 of this embodiment, a semiconductor material with abandgap larger than the optical input can be selected for the layers310, for example Al_(x)Ga_((1-x))As with x between 10% and 35% or InGaP.Alternatively, GaAs can also be selected for the layers 310, but thenthe value of the thickness of the GaAs layer 310 has to be subtracted inthe calculations of the above values of d1 to d5 because the opticalinput 100 will also be absorbed in the layers 310. The other layers 302,304, 306, 308 of the connecting elements 300 are all selected from agroup of semiconductor material with a bandgap larger than the opticalinput such as for example Al_(x)Ga_((1-x))As with x between 10% and100%, InGaP, or AlInP. In addition to the above, graded indexinhomogeneous dielectrics which can be fabricated for examples usingoblique angle deposition or PECVD plasma techniques can also providegood anti-reflective coatings by themselves or can be used as part of astack of layers. In all of the above the thickness(es) of theanti-reflection layer(s) can be estimated or calculated using multiplereflections algorithms but in general the layers' thickness will be afraction of the wavelengths of interests namely between 5 nm and 200 nmdepending on the designs and number of layers used.

As will be understood by the skilled worker, the exposed portion of thewindow layer 14, or a portion of the front surface 102 having had thecontact layer 12 removed cap therefrom (the light-input portion of thefront surface 102) can be covered by an antireflection coating made ofany suitable material such as, for example, TiO₂, Al₂O₃, SiO₂, Sb₂O₅,Ta₂O₅, SiN_(x), MgF₂, ZnSe, ZnS, zirconium oxide, zirconium dioxide orIndium-Tin-Oxide layers, or any other suitable combination of two ormore of these layers, or similar dielectric layers, typically chosenwith a combination of index of refraction that tend to minimize thereflections over the wavelength range of interest by essentiallyproviding progressive steps in the index of refraction when going fromthe phototransducer window 14 to the surrounding medium which istypically air, an encapsulant medium, beam shaping optics or acombination of the above used to guide the optical input into thephototransducer and further protect its front surface 102 or othersensitive layers. A bi-layer combining a low index of refraction and ahigh index of refraction from a choice of the above dielectric typicallyprovides good anti-reflective properties as would be known by a skilledworker. For example one of the following bi-layers can used TiO₂/Al₂O₃,SiO₂/Sb₂O₅, SiO₂/Ta₂O₅, Al₂O₃/Sb₂O₅, SiO₂/SiN_(x), MgF₂/ZnSe,Al₂O₃/Ta₂O₅, MgF₂/ZnS, SiO₂/TiO₂, or Indium-Tin-Oxide layers combinedwith some of the above dielectrics.

FIG. 9 shows the results for the embodiment of a transducer asillustrated in FIG. 2, comprising a base 18 using a p-type GaAs, withthe thickness 120 of 3.5 microns, but comprising no connecting elementsin the base 18. An example of data of an I-V measurement is plotted incurve 410 for a device temperature of 25 C. The I-V curve 410 isobtained for an illumination intensity of 1366 W/m², with an embodimentincorporating GaAs substrate 24, a GaAs buffer layer 22, a GaInP backsurface field 20, a n-type GaAs emitter 16, a GaInP window 14, a duallayer antireflection coating deposited on the front surface 102, andpatterned contact and metal layers 12 and 10 to extract the photocurrentand photovoltage from the phototransducer. The I-V measurement is takenbetween the top metal 10 and a metal contact applied to the substrate24. The measured I-V data is modelled using the diode equationI(V)=I_(ill)−Io [exp(eV/nkT)−1], where I(V) is the diode current at theapplied voltage V, IiII is the current from the illumination, Io is thesaturation current of the photodiode, V is the applied voltage, n is thephotodiode ideality factor (which can also be called the n-factor), k isthe Boltzmann constant, and T is the temperature (here 25° C.). Thediode parameters can be extracted from the measured data obtained onsuch a GaAs phototransducer. These diode parameters can then be used tomodel phototransducers of the embodiments incorporating connectingelements but otherwise based using the same type of materials and devicegrowth and fabrication conditions. For example curve 412 in FIG. 9 showsthat the model fits well the experimental data when using a saturationcurrent Io˜8.8E-12 mA/cm², and a n-factor˜1.45. These diode parametersgive for the curves 412 or 410 of FIG. 4: Voc˜1.03V, a FF˜84.6%.

The device parameters extracted from FIG. 9 are then applied to model aphototransducer incorporating 4 connecting elements as depicted in FIG.2 and described herein, and to predict the performance of thatphototransducer under various conditions. For example, FIG. 10 showssuch an I-V curve for a phototransducer with a base 18 of thicknesst˜4.0 microns incorporating 5. GaAs base segments s1, s2, s3, s4, and s5which would each absorbed 19.8% of an optical input 100 impinging thefront surface 102 with a wavelength of 830 nm. The detailed requirementsof the various layers for this embodiment have been described above. TheI-V curve 510 of FIG. 10 is produced for an optical input of 1 Watt/cm²,for a phototransducer having a quantum efficiency of 97% at 830 nm. TheI-V curve 510 has its maximum power point near the knee of the curve 512and the analysis of the I-V yields the following performance metrics:Voc˜5.632V, Isc˜130 mA/cm², FF˜86.1%, FFv˜89.2%, FFi˜96.6%, Vmax˜5.022V,Imax 125.4 mA/cm², Pmax˜630 mW/cm2, and with a conversion efficiency ofEff˜63%.

The deposition or the growth of the various semiconductor layersdescribed in the embodiments presented and described in relation to FIG.2 and FIG. 7 can be carried out through any suitable means ofsemiconductor growth such as: metal organic chemical vapor deposition(MOCVD), chemical beam epitaxy (CBE), molecular beam epitaxy (MBE),solid phase epitaxy (SPE), hydride vapour phase epitaxy or by othersimilar hybrid systems or combinations thereof. The growth parameterscan be optimized for the various layers of the embodiments, for exampleto maximize the device performance or its manufacturability. The growthparameters and growth conditions that can be optimized include, forexample, the growth temperature, the pressure of the various gases usedto grow the layers, the ratio of those pressures (for example the III/Vratio when growing III-V semiconductor layers), the alloy composition,the residual strain, the growth rate, the doping or co-doping of thevarious layers, the use of surfactant gases, the use of annealingcycles, etc.

The epitaxy of such layer can be done in a process using a single waferor multiple wafers per run. Each wafer can between 25 mm and 450 mm indiameter depending on the type of substrates used and their commercialavailability. The off-cut angle of the substrate, which can also becalled the misalignment angle of the substrate, can also affect thegrowth conditions and the quality of the layers and can therefore beadjusted to optimize the device performance. For example, the growth canbe done on (100), (110), or other surfaces, and off-cut angles towardvarious crystallographic planes can be used with angles varying between0 degree and 40 degrees. The typical precision and control over thethickness, the composition, and the doping of the semiconductor layersgrown using the above epitaxy techniques are typically well withintolerable variations of the specifications to achieve the desiredembodiments as described in the present disclosure.

Furthermore, the quality of the layers of the various embodiments, andthe quality of the semiconductor material comprised in these layers, canbe determined by probing, post-growth, the optical properties,electronic properties, or both, of the arrangement such as the layersshown in FIG. 2. This can be achieved by measuring the optical spectralresponse (quantum efficiency) of the photocurrent (PC) or photovoltage(PV), photoluminescence (PL), electroluminescence (EL), time-resolvedphotoluminescence (TRPL), time-resolved photo-current (TRPC),electron-beam induced current (EBIC) measurements, or othercharacterization techniques as described in the present disclosure orother techniques known to one skilled in the art. The characterizationcan be performed on calibration or validation growth runs, or on theentire heterostructure, or on some of the layers comprised within thephototransducer, or with completed devices, including the connectinglayers of FIG. 7. Such techniques can be used to determine the minoritycarrier lifetimes, the thicknesses of the layers, the optical paths ofthe layers, and other optoelectronic properties relevant to the deviceperformance. It can also be used to reveal and/or assess for example thedependence of the phototransducer performance on the number ofconnecting layers (c1, c2, c3, c4, etc) and the position of thoseconnecting layers (d1, d2, d3, d4, etc). Furthermore, reflection highenergy electron diffraction (RHEED) and optical reflectance of thetransducer of FIG. 2 for example, can also be used to obtain surfaceroughness and morphology information of the various layers during orafter the growth of the layers of FIG. 2 or FIG. 7. Such in-situtechniques and other optical techniques can be used to give a real-timefeedback on the optical properties of the layers and/or on themorphology as it progresses, and can be used to determine the quality ofthe layers. The in-situ techniques include reflection of photon (light)or electron beams during epitaxial growth of the arrangement of FIG. 2for example. These techniques can also provide a measurement of thecurvature of the semiconductor wafer to evaluate if there are any strainbuild-up or strain-relaxation events occurring during the epitaxialgrowth. Growth conditions/parameters can be adapted as a function of thein-situ monitoring to compensate for any undesired effects observedduring growth. For example, optical probing can be performed bymonitoring the reflection of an optical beam using a probe beam withsimilar wavelength as the optical input 100 to be used for theapplication of interest. Alternatively, the optical probing can be doneusing a wavelength that takes into consideration the different index ofrefraction of the semiconductor layers at the growth temperature duringthe epitaxy. The target parameters for the various layers can then beadjusted during the growth to achieve the desired target values for thethickness of the various based segments (s1, s2, s3, s4, etc) asmonitored using such in-situ techniques.

FIG. 11 shows the expected performance variations of a phototransduceraccording to the present disclosure when the wavelength of the opticalinput is varied from the optimum design value. Plot 610 of FIG. 11displays, for various optical input wavelengths, the relativeperformance of a phototransducer with four connecting elements asillustrated and described in FIG. 2. The phototransducer of FIG. 11 isdesigned for an optical input wavelength of 830 nm. The plot 610displays a performance maximum 612 at an optical input wavelength of 830nm. At an optical input wavelength away from the targeted 830 nm therelative performance decrease slightly, but it still exceeds 80% of theoptimal performance if the optical input wavelength is changed to ˜800nm and still exceeds 90% if the optical input wavelength is changed to˜850 nm.

FIG. 12 shows the expected performance variations of a phototransduceraccording to the present disclosure when the thicknesses of the baselayer segments are varied from the optimum design value. Such thicknessvariations for example could occur from variations in different regionsof a wafer, or from variations from wafers to wafers within a samegrowth, or from wafer growth to wafer growth. Plot 710 of FIG. 12 shows,as a function of the relative thickness error, the relative performanceof a phototransducer with four connecting elements as illustrated anddescribed in FIG. 2. The plot 710 shows a performance maximum 712 atzero thickness error (i.e. optimum design parameters as described in theabove embodiments). As the thickness error is increased on either sideof the optimum value (i.e. thinner layers or thicker layers) therelative performance decrease slightly, but it still exceeds 80% of theoptimal performance if the magnitude of the thickness reaches 15%.Production epitaxy reactors can typically be controlled to values muchbetter than +/−15%, and often well within only a few percent.

FIG. 13 shows the phototransducer conversion efficiency as a function ofthe power of the optical input 100 for a phototransducer with fourconnecting elements as illustrated and described in FIG. 2. Plot 810shows a non-linear dependence which increases for increasing powers ofthe optical input. At 100 mW input, the efficiency if Eff˜58% and itincreases to Eff˜64.5% at about 2 W of optical input.

As is shown at FIG. 14, the increase in performance observed in FIG. 13with increasing optical input power mainly originates from the increasein photovoltage with increasing optical input power. For example, FIG.14 plots the phototransducer Voc as a function of the power of theoptical input 100 for a phototransducer with four connecting elements asillustrated and described in FIG. 2. Plot 910 shows a non-lineardependence, similar to FIG. 13, which increases for increasing powers ofthe optical input. At 100 mW input, the open circuit voltage isVoc˜5.22V and it increases to Voc˜5.76V at about 2 W of optical input.

The above embodiments exemplify the major performance parameters andphototransducer behaviors for the material parameters which correspondto the values obtained with GaAs as measured and not necessarilyoptimized. One skilled in the art will realize that the performance canbe further optimized through device development and manufacturingprocesses and with further optimization of some design parameters. Forexample the ideality factor for the various base segments can beoptimized from the material quality and various design aspects. Thelatter can lead to higher values of Voc. Furthermore, the optimizationof the minority carrier lifetimes would also improve the Voc values.Similarly the FF can be optimized by ensuring that the layer design isfavorable to an efficient current extraction and that the sheetresistances, contact resistances, and other series resistances areminimized. The optimization of the bandgap energies used for thedifferent semiconductors in the various layers will help minimizing thethermalization losses of the photons from the optical input absorbed bythe semiconductors.

The phototransducer of the present disclosure can be realized on asubstrate which contains an active p-n (or n-p) junction capable ofdetecting photons in the optical input which could be at a differentwavelength. That is the optical input 100 can have two sources, a firstoptical power source which will be converted by the base segments asdescribed in the above embodiment, and simultaneously a secondaryoptical signal which will be detected by a p-n junction constructedwithin the substrate layer 24. The monolithically integratedphototransducer of this embodiment can be used for example forapplications which transmit simultaneously a data signal and opticalpower. For example, the embodiment can comprise a GaAs base layer 18with connecting elements as illustrated in FIG. 2 to produce aphototransducer and the embodiment can further comprise a Ge substratethat includes a p-n junction. The Ge p-n junction can be obtained bydiffusion during the growth of the III-V layers on the Ge substrate, forexample by diffusing group V atoms to create an n-type emitter into ap-type Ge substrate which constitute the base. The wavelength of thefirst optical power source preferably has a wavelength comprised between500 nm and 880 nm, or between 750 nm and 880 nm. The wavelength of thesecondary optical signal has a wavelength comprised between 1250 nm and1700 nm, or between 1300 nm and 1650 nm. The p-n junction formed in theGe substrate is adjacent to phototransducer on the side opposite to thefront surface. To optimize the material quality the GaAs base layer 18in this embodiment a small fraction of about 1.1% indium can beincorporated (i.e. In_(x)Ga_((1-x))As with x˜1.1%) to insure the basesegments and the other layers grown on the substrate remainlattice-matched to the Ge lattice constant. In such an embodiment, thetransducer is electrically connected to transducer circuitry through afirst set of electrical contacts and, the Ge p-n junction iselectrically connected to data processing circuitry to process the datasignal detected at the Ge p-n junction.

FIG. 21 shows an embodiment of a transducer and data receiver unit thatreceives, in the optical input, optical power at a first wavelength(first energy) and a data signal at another wavelength (energy). Theoptical power is detected at the transducer portion, which comprises thesemiconductor emitter layer 16 and at the base 18, which includes thesemiconductor base layers 130, 132, 134, 136, and 138. The data signalis detected at p-n junction 3000, which is formed in the Ge substrate24.

Photo carriers (electrons and holes) generated in the transducer of thepresent disclosure are collected by electrical contacts. In anembodiment, where the emitter is an n-type semiconductor, electronsarrive at the emitter and then flow laterally through the emitter layerbefore reaching a metal electrical contact. In another embodiment, wherethe emitter is a p-type semiconductor, holes arrive at the emitter andthen flow laterally to the metal electrical contact. In both cases, theemitter can be doped in the range of 5×10¹⁸ to 2×10²⁰ in order tominimize resistive losses. Doping at too high concentrations however canreduce the carrier mobility and hence the layer conductivity as well asthe minority carrier lifetime; a compromise can be made to optimize theoverall performance of the transducer. Additionally, it can beadvantageous that the metal electrical contact be ohmic in nature have alow resistance per square of 1×10⁻⁵ ohm-cm² or lower.

In cases where the illumination area of the photo transducer is largeand carriers have to travel long lateral distances to the electricalcontact, typically from a few hundred microns or more, and/or wherelarge current are generated, the resistive losses from current flowingthrough the emitter layer can be significant. It is common practice inthese cases to use a metal electrical contact that has a metal grid linepattern designed to reduce the lateral distances traveled by thecarriers and to minimize the emitter resistance. Prior art grid linesare typically between one and 15 microns wide and are separated by 50microns or more depending on the current densities generated for aparticular application. Grid lines on the other hand, will block(shadow) some of the incident light thereby reducing the total currentand efficiency of the transducer. The design optimization thereforerequires a careful balance between the emitter doping levels, the gridline pattern and the metal contacts.

An alternative to grid lines is to form a transparent conductive film(TCF) on the emitter layer or on the window layer. The incident lightcan then propagate freely through the TCF and carriers generatedunderneath and travelling back into the emitter can make use of thetransparent electrode as a parallel path to reach the metal contact.This embodiment eliminates shadowing of the incident light from gridlines and helps to minimize the lateral resistance. The TCF, which is atransparent electrode, can be made of indium tin oxide or ITO; fluorinedoped tin oxide or FTO; zinc oxide or ZnO; aluminum doped zinc oxide orAZO; indium doped cadmium oxide; binary compounds of metal oxides;organic films using carbon nanotubes and graphene; polymers such aspolyacetylene, polyaniline, polypyrrole, polythiophenes; andpoly(3,4-ethylenedioxythiophene) and their derivatives.

In other embodiments the transparent electrode can be made of highlydoped semiconductor materials, with doping typically comprised between10¹⁸ to 10²⁰ cm⁻³, and with a bandgap higher than the photon energy ofthe incident light to allow complete transmission of the incident light.The semiconductor materials in these embodiments can include III-V orII-VI compound semiconductor materials including but not restricted tothe same materials listed earlier for the passivating window 14 of FIGS.1 and 2.

FIG. 15 shows a top view of transducer 2000 of the present disclosurethat comprises grid lines 2002 separated from each other by a distanceof 325 microns. FIG. 16 shows a top view of transducer 2004 of thepresent disclosure that comprises grid lines 2006 separated from eachother by a distance of 425 microns. FIG. 17 shows a top view of atransducer 2008 of the present disclosure that is free of gridlines butthat comprises a TCF 2010.

FIG. 18 shows the averaged short circuit current (Isc) for thetransducers 2000, 2002, and 2004 of FIGS. 15, 16, and 17. FIG. 19 showsfill factor (FF) data for transducers 2000, 2002, and 2004 of FIGS. 15,16, and 17. An incident beam of light with wavelengths centered around(substantially at) 850 nm was used to acquire the data. All devices hada transparent electrode made of InGaP doped to a doping level of 10¹⁸cm⁻³ and had five base segments. FIG. 18 a slightly higher Isc for thegridless transducer 2008, which is as expected. FIG. 19 shows that theFF is substantially the same for all the transducer 2000, 2004, and2008. The FF is directly affected by the presence of any seriesresistance in the circuit, including that of the emitter and metalcontact. The values of FF of FIG. 19 would be lower for the transducer2008 without the presence of the transparent conductive InGaP electrode.

The transducer of the present disclosure has a well-defined and widedynamic range response in voltage and current with respect to theoptical input power. The current and Isc have a linear dependence on theinput optical power and, Voc has a logarithmic response. The transducerof the present disclosure can therefore be used in applications wheremonitoring the intensity of a highly focused beam of monochromatic lightis needed. Examples of applications may include continuous wave (CW) andpulsed laser beams and focused light from LED's or other monochromaticsources of light. The use of a design with no grid lines is particularlyuseful when accurate power monitoring and measurements is required. Thisis because focused light beams may have beam diameters or spot sizescomparable to the widths and spacing of the grid lines, therebyintroducing considerable errors from shadowing if the beam falls on orin between grid lines. Furthermore, high power density light beams canbe problematic for prior art technologies using photo-diodes withlimited power ratings. These will often require the use of additionalbulky beam splitters and attenuators also sensitive to high powerdensities, to quench and attenuate the incident power of the beam tomonitor.

In the application for which the phototransducer of the presentdisclosure is used as a power meter device, the phototransducer can beequipped with a compact voltage readout circuit to compare the measuredphototransducer Voc to a calibration table containing the relationshipbetween Voc and the input optical power. For example, the latter datacan be stored in memory and a programmable logic circuitry can be usedto output, on a display for example, the detected power. The storedinformation can contain the photo-voltage response data for variousinput wavelengths as illustrated in previous figures. The data for agiven wavelength (here 830 nm) is illustrated in FIG. 20, where curve1010 shows the calibrated relationship between Voc and the input opticalpower, vertical line 1020 is an example of measured Voc which could bedetected with the phototransducer, and the horizontal line 1030 whichcrosses the intersection of curve 1010 and line 1020 is thecorresponding detected power, here expressed in Watts for FIG. 20.

As will be clear for one skilled in the art, there are several benefitsin using the phototransducer of the present disclosure as a laser powermeter. The benefits are not limited to, but include the following: asmentioned above, the optical input surface can be designed without anymetal gridlines. The metal gridlines can absorb and scatter the laserlight, giving errors in the measurements, possible increased safetyhazards, and possibly leading to catastrophic failure of the power metersensitive area in the case of high power input laser light which getsabsorbed by the metal gridlines. Avoiding the gridlines is thereforeadvantageous. Furthermore, because of its construction based on amonolithically stacked designed with multiple base segments, theresponse phototransducer of the present invention is notposition-sensitive. That is the laser light can impinge differentpositions on the optical input areas without affecting significantly thedetected power. Moreover, because of the very high optical to electricalconversion efficiency of the phototransducer of the present invention,less wasted energy would be deposited in the optical active detectinghead of such a power meter, therefore reducing the risk of damaging thedevice with the input light source.

It will also be clear for one skilled in the art that such a power meterutilizing the phototransducer of the present disclosure could beoperated, without departing from the scope of the present disclosure, inopen circuit mode (Voc-mode), in short-circuit mode (Isc-mode), inmaximum power point mode or voltage of maximum voltage power (Vmp-mode),or at other positions on the operational I-V curve of thephototransducer. As will be understood by the skilled worker, it can beadvantageous to operate the device near the maximum conversionefficiency point (near Vmp) to convert as much input optical power intoelectrical power. The converted power will avoid the need for evacuatingexcessive thermal loads, and the optical power which is converted intoelectrical power from the optical input can be stored, in rechargeablebatteries for example. The stored power can then be used to run theelectronic circuitry of the power meter. The latter configuration can beadvantageous to reduce or avoid the need for recharging the power meterunit into an external source such as a wall plug, or for pluggingrechargeable batteries into an external source such as a wall plug. Forexample, a compact handheld power meter, with no need for external powersource other than a light source (e.g., laser source) to be measured,can be constructed with the phototransducer of the present disclosureequipped simply with a rechargeable battery, a display, and a readoutand logic circuitry to convert the measured data into displayed reading.The display can be a digital liquid crystal display or an analog dial.The power meter thus constructed could draw its power directly from thesource to be measured if the laser power is high-enough. In the case forwhich small laser powers need to be measured, the power meter could berecharged externally from an electrical power source or an optical powersource. FIG. 22 shows an example of such a power meter 4000 thatcomprises a transducer 4002 of the present disclosure, a readoutcircuitry 4004 that receives an electrical output from the transducer4002 (e.g., the output voltage of the transducer 4002), a processor 4006operationally connected to the readout circuitry 4004. The processor4006 is configured to provide an electrical signal to a chargingcircuitry 4008, which charges a battery 4010. A display 4012 inoperationally connected to the processor 4006; the display can show avalue of the optical power to which the transducer 4002 is subjected.

In other embodiments, the present disclosure provides a compact handheldpower meter, with no need for external power source other than a lasersource to be measured. The power meter in question can comprise thephototransducer of the present disclosure equipped with a rechargeablebattery and readout and logic circuitry configured to obtain data and toprovide the data wirelessly to a mobile device equipped with software toread and display, on the mobile device, the data.

In general, and in the context of the present disclosure, two componentsare “electrically connected” when an electrical change caused by oraffecting one (such as a change in voltage or current) can result in anelectrical change in the other or, when an electrical signal sent by onecan be received by the other. The two components need not be directlyelectrically connected (that is, there may be other elements interposedbetween them).

At least one embodiment is disclosed and variations, combinations,and/or modifications of the embodiment(s) and/or features of theembodiment(s) made by a person having ordinary skill in the art arewithin the scope of the disclosure. Alternative embodiments that resultfrom combining, integrating, and/or omitting features of theembodiment(s) are also within the scope of the disclosure. Wherenumerical ranges or limitations are expressly stated, such expressranges or limitations may be understood to include iterative ranges orlimitations of like magnitude falling within the expressly stated rangesor limitations (e.g., from about 1 to about 10 includes, 2, 3, 4, etc.;greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example,whenever a numerical range with a lower limit and an upper limit isdisclosed, any number falling within the range is specificallydisclosed. Additionally, the use of the term “substantially” means+/−10%of a reference value, unless otherwise stated. As an example, thephrase: “voltage A is substantially the same as voltage B” means thatvoltage B is within +/−10% of voltage B.

The embodiment of the transducer shown at FIG. 2 is epitaxiallymanufactured and has a monolithic vertical architecture that comprisesportions having different compositions of semiconductor materials. Assuch, this type of transducer architecture can be referred to as avertical epitaxial heterostructure architecture (VEHSA). As will beunderstood by the skilled worker, the composition of semiconductorcompound materials will determine, at least in part, the bandgap energyof a given semiconductor compound and, manufacturing semiconductorcompounds to obtain a target bandgap energy is well known in the art.

For some embodiments, the inventors have discovered that manufacturingsuch VEHSA transducers with a bandgap energy of its constituentsemiconductor materials slightly larger than the energy of the photonsof the optical light source used to operate the transducer, enables thetransducers to have an efficient light to electricity conversionefficiency. Such embodiments can be referred to as sub-bandgapphototransducers or subgap phototransducers. Further, the inventors havediscovered that given a VEHSA phototransducer with a set bandgap energy,selecting an optical light source that has a photon energy smaller thanthe bandgap energy can result in efficient light to electricityconversion efficiency.

The inventors have also discovered that manufacturing such VEHSAtransducers with a bandgap energy of its constituent semiconductormaterials slightly smaller than the energy of the photons of the opticallight source used to operate the transducer, enable the transducers tohave an improved light to electricity conversion efficiency compared toother heterotructure designs or material configurations.

The inventors have found experimentally that illuminating a VEHSAtransducer with light having a photon energy that is within a smallenergy difference equal to the bandgap energy of the VEHSA transducerproduces the best conversion efficiency of photons to electrons. Thesmall energy difference is, in some cases, of the order of the phononenergy for the VEHSA transducer material. That is, the inventors havediscovered that VEHSA transducers can also be effective and, in somecases, more effective at converting the incoming photon energy based onvarious subgap absorption phenomena and other linear or non-lineareffects, some of which are described below, than by, as is usually done,converting light having a photon energy greater than the bandgap energyin other device configurations or even in some cases of the VEHSAtransducer operated with photon energy greater than the bandgap energy.

As will be described below, embodiments of the VEHSA transducers of thepresent disclosure have greater light to electricity conversionefficiency when the photon energy (of the light beam) is less than thebandgap energy of the transducers in question. The higher efficiencyallows the light sources to be operated at lower power than theynormally would for transducers having a bandgap energy less than thephoton energy. Advantageously, running the light sources at lower powerincreases their useful lifetimes.

Further, for transducers made of typical semiconductor materials, suchas GaAs, but possibly also InP, InGaAs, AlGaAs, GaInP, GalnAsP, Si, Ge,InAs, InSb, AlSb, or combinations of such alloys, that have a bandgapenergy of 1.42 eV in the case of GaAs for example, light sources havinga photon energy less than the bandgap energy can be generally lessexpensive to manufacture than light sources that have a photon energylarger than the bandgap energy, particularly for the case of GaAs forexample.

Furthermore, operating the transducers at a photon energy lower than thebandgap energy allows for the transducers to run cooler than they dowhen operating at photon energies greater than the bandgap energy. Thiscan prolong the lifetime of the transducers. This can be the case forexample because less thermalization energies of the photo-excitedcarriers are present when operating the device in these conditions, andthere is therefore less dissipated heat.

As will be described below, embodiments of the transducers of thepresent disclosure exhibit greater light to electricity conversionefficiency when the photon energy (of the light beam) is, in some cases,less than the bandgap energy of the transducers in question. This allowsthe light sources to be operated at lower power than they normally wouldfor transducers having a bandgap energy less than the photon energy.Advantageously, running the light sources at lower power increases theirlongevity.

FIG. 23 shows an example of the valence band 5000 and the conductionband 5002 of a semiconductor material, where the vertical direction inthe figure represents the energy of the charged carriers within thesemiconductor. One with ordinary skill in the art would thereforeunderstand that the difference in energy between the conduction band5002 and the valence band 5000 is often referred to as the forbiddengap, the energy gap, or the bandgap of the semiconductor material,having a value denoted by E_(gap) or E_(g), which is a characteristicproperty of the semiconductor material. With the value of E_(gap), athreshold wavelength can be associated to the bandgap energy by therelationship λ_(threshold)=hc/E_(g), where c is the speed of light and his Planck's constant. In general, and as will be readily understood bythe skilled worker, for optical wavelengths λ_(input) longer than thethreshold wavelength, λ_(input)>λ_(threshold), the absorption is weak.

FIG. 23 also shows energy levels 5004 near the valence band 5000 andenergy levels 5006 near the conduction band 5002. These energy levels5004 and 5006 are within the forbidden gap, i.e. between the valenceband 5000 and the conduction band 5002 and can have differentdensity-of-states depending on the origin of the energy levels 5004 and5006 within the gap. Energy levels near the conduction band 5006, suchas energy levels 5006 can be related to donor impurities in thesemiconductor material (semiconductor alloy) and are often referred toas donor levels. Similarly, energy levels 5004 near the valence band5000 can be related to acceptor impurities in the semiconductor materialand are often referred to as acceptor levels. Quantum effects insemiconductor heterostructures containing nanostructures such as quantumwells, quantum wires, or quantum dots can also effectively generatesimilar discrete levels and finite density of states within the energygap.

Such energy levels within the forbidden gap can give rise to absorptiondue to subgap transitions. For example, FIG. 23 shows a carrier (chargecarrier) 5008 in the acceptor level 5004. An incoming photon having anenergy equal to or greater than the transition energy between theacceptor level 5004 and the donor level 5006, depicted by the upwardarrow 5010 between the acceptor level 5004 and the donor level 5006, canbe absorbed in this photocarrier excitation process. Similarly FIG. 23shows a valence band 5000 to donor level 5006 transition 5012 and anacceptor level 5004 to conduction band 5002 transition 5014. There arevarious transitions that can participate in subgap photon absorption.Indirect transitions in momentum space within the heterostructure canalso give rise to absorption at energies lower than the bandgap.

Another example of subgap photon absorption is associated to the Urbachenergy tail. As is the case with the Urbach tail, some of the subgapabsorption can depend on the temperature of the semiconductor materialand/or on the intensity of the optical beam illuminating the transducer.

When the absorption varies substantially linearly with the intensity ofthe input beam, the effect is said to be linear, but when the absorptionvaries non-linearly, then a non-linear effect can appear in the deviceproperties and performance. When the absorption saturates at higherintensities, absorption saturation or bleaching effects can be present.In other cases the absorption can increase with the input beam intensityleading to non-linear increase in performance with input intensity.

This can arise for example when the density-of-state of the levels(e.g., impurity state levels) participating in the transition and theoptical input beam intensities are such that a substantial population ordepletion of some levels can be achieved by optical pumping. The opticalpumping can be indirect through an effective electrical forward biasingof the diode structure generated from the illuminated device (opticaltransducer). For example, most devices have opaque metal contacts,preventing substantial illumination under the metal in some area of thedevice, so by illuminating the non-metalized area of the device, themetalized area becomes effectively forward biased. In forward bias thediode will typically generate electroluminescence or possibly laseremission. Level saturation and non-linear effects can also occur whenlaser transitions or stimulated emission are present, or when apopulation inversion is achieved with electrical or optical pumping.

The sub-bandgap absorption can also be achieved in multiple steps. Forexample, in FIG. 23, a two-step process (transition) is shown involvinga first step 5015 and a second step 5016. There are many possiblecombinations: the first step 5015 can be smaller than the second step5016, or vice versa. The first step and the second step can take placesimultaneously, or at different times, via virtual states, or via thepopulation of real states such as state 5004. But in all cases, if themultiple steps added together have an energy equal or greater than theenergy gap of the semiconductor, then a carrier can be promoted from thevalence band 5000 to the conduction band 5002 by such multiple-stepprocesses. Here step 5015 is depicted as having a smaller energy thanstep 5016 (e.g. a photon absorption transition) and could represent aphonon transition from the valence band 5000 and the correspondingphonon energy. For example, transverse optical (TO), longitudinaloptical (LO), transverse acoustic (TA), and longitudinal acoustic (LA)phonons can participate to complement the input photon energy 5016having a sub-bandgap energy (i.e. a wavelength longer than thewavelength corresponding to the threshold wavelength associated to thebandgap energy λ_(nput)>hc/E_(g)).

A single phonon energy can be, for some semiconductor compounds, between˜0 meV and 75 meV. Whereas the subgap absorption is typically low ingeneral for most semiconductors at low intensities, at higher inputintensities, and at finite temperatures such as room temperature, it canbe expected that such subgap absorption effects will become moreefficient. That is because the probability for a population buildup ofintermediate states increases, or the scattering rates for phonon ormulti-phonon interaction, or multi-carrier interaction, such as Augerscattering, are typically higher in these conditions.

Furthermore, in a semiconductor material, the energy gap decreases withtemperature. The change in the temperature will change the energydifference between the optical input, e.g. 5016 and the available energytransitions 5010, 5012, 5014, E_(gap), etc. Therefore, the portion ofthe energy of the input beam that is not converted to electricity by thephototransducer is dissipated in heat, which can increase thetemperature of the semiconductor and shift its bandgap closer to thethreshold for absorption of the input wavelength by interbandtransitions or by sub-bandgap absorption effects as depicted in FIG. 23.

In view of the effects described above, the compositions of the VEHSAphototransducer can be modified to optimize the subgap performance withrespect to efficiency. The optimization can be based, among others, onthe absorption characteristics for the desired wavelengths for theapplication of interest. Also, an embodiment optimized for operation atother interband absorption (such as for example the wavelength rangebetween 800 nm and 880 nm for GaAs based devices) can be used also forsubgap absorption. The performance can depend on the VEHSAphototransducer configurations and the conditions of operation. Forexample FIG. 23 can be applied for a GaAs VEHSA phototransducer forsubgap absorption in the wavelength range between 880 nm and 1000 nm andpreferably between 910 and 930 nm for the optical input represented bythe transition 5016. For this embodiment the base semiconductor materialcan be chosen to be preferably GaAs, or Al_(x)Ga_((1-x))As with xcomprised between 0% and 5%. For p-type GaAs base, as illustrated inFIG. 2 with a thickness (120) of t=4 microns, base segment values ofd1=192 nm, d2=438 nm, d3=784 nm, and d4=1365 nm can be selected todivide the base into 5 segments, s1, s2, s3, s4, and s5. The base of theVEHSA structure can be divided in a different number of segments, forexample 8 segments, for the results shown in FIG. 24 (plot 5026) andFIGS. 25A and 25B (plot 5030). The number of base segment is preferablybetween 1 and 100. However, there can be any number of segments, withoutdeparting from the scope of the present disclosure.

The thicknesses of the base segments of a subgap VEHSA device can beoptimized by calculating or by measuring the amount of subgap absorptionat different wavelengths and at different values of input beamintensities. It can be preferable to independently measure p/n junctionsof different thicknesses to quantify the subgap absorption. For example,FIG. 24 shows the input power dependence of the short circuit currentmeasured from single GaAs p/n junctions of different thicknesses. At lowoptical input powers (e.g. for P_(in) smaller than 1 W), theshort-circuit current is very low for all the different p/n junctionthicknesses shown in FIG. 24. However, as the input power is increased,the short circuit current first increases and then tends to saturate.For example, in FIG. 24, the p/n junctions of various thicknesses allproduce on the order of 1 A of short-circuit current for input powersgreater than 3 W. Plot 5018 is for a GaAs p/n junction having athickness of 2636 nm; plot 5020 is for a GaAs p/n junction having athickness of 581 nm; plot 5022 is for a GaAs p/n junction having athickness of 346 nm; plot 5024 is for a GaAs p/n junction having athickness of 192 nm; and plot 5026 is for a GaAs VEHSA structure with 8base segments having all together a total thickness of 4500 nm andhaving its thinnest base segment with a thickness of 126 nm.

Since p/n junctions with various thicknesses tend to give comparablephoto-current values at high intensities of subgap excitations, theinventors have discovered that it can be advantageous to operatephototransducer devices based on the VEHSA design under theseconditions. This is clearly demonstrated by plotting the subgap opticalto electrical conversion efficiencies as a function of the optical inputpower. This is shown in FIG. 25A for GaAs-based devices excited with abelow-gap optical power of 909 nm. In FIG. 25A, plot 5028 is for theGaAs p/n junction that has a thickness of 2636 nm; plot 5030 is for theGaAs VEHSA structure with 8 base segments having all together a totalthickness of 4500 nm and having its thinnest base segment with athickness of 126 nm; plot 5032 is for a GaAs p/n junction that has athickness of 581 nm; plot 5034 is for the GaAs p/n junction that has athickness of 346 nm; and plot 5036 is for the GaAs p/n junction that hasa thickness of 192 nm. All devices in FIG. 25A exhibit low efficienciesat lower intensities (powers <1 W for example). The thicker p/n junctionassociated to plot 5028 reaches a maximum peak efficiency of ˜24% at anintermediate power of ˜2.5 W, but the VEHSA phototransducer with 8 basesegments, associated with plot 5030, dramatically outperforms all theother configurations that are not based on a VEHSA design. In FIG. 25A,the VEHSA photo transducers have been demonstrated to exhibit a subgapconversion efficiency as high as ˜36% and converted powers reaching upto 1.4 W in electrical power output under these conditions.

FIG. 25B shows the same data as in FIG. 25A but plotted as a function ofpeak power intensity. For plot 5030, which is associated to a GaAs VEHSAstructure with 8 base segments, a conversion efficiency of about 10% isobserved for a power intensity of about 300 W/cm² and over 35% for apower intensity of 800 W/cm². In fact, the inventor has observed, forGaAs transducers, useful conversion efficiencies are obtained forsub-gap optical inputs for power intensities ranging from 100 W/cm² to2000 W/cm². Similarly, while phototransducers based on othersemiconductor materials will have sub-gap absorption at differentwavelength range, as discussed above, it can be expected that similarsub-gap conversion efficiency values can be obtained based on the abovedesign considerations.

The short-circuit current for subgap excitation at high intensities isnon-linear as shown by the nonlinear plots of FIG. 24. The efficiency isclearly increasing with the input power of the subgap excitation asshown by the plots of FIG. 25A. This means that the responsivity,defined as the photocurrent divided by the input power in A/W, of theVEHSA phototransducer is increasing with the excitation power. For VEHSAstructures, the responsivity can be multiplied by the number of basesegments in order to directly compare the measured responsivity valuesof the VEHSA devices to the values of other devices such as single p/njunction devices.

For an optical input at a wavelength λ_(input), the maximum theoreticalresponsivity is given by R=λ_(input)/1239.85*QE, where λ_(input) isexpressed in nm and QE is the quantum efficiency of the device, and R isthe responsivity in A/W. The maximum theoretical responsivity is shownin FIG. 26 by the line 5038 for QE=92.5% and an optical input wavelengthof 909 nm. FIG. 26 also shows, for a VEHSA transducer having twelve GaAssegments, the measured responsivity for the input powers of 1 W (200W/cm²) at plot 5040, 2 W (200 W/cm²) at plot 5042, 3 W (600 W/cm²) atplot 5044, 4 W (800 W/cm²) at plot 5046, and 5 W (1000 W/cm²) at plot5048. Clearly the responsivity of the subgap VEHSA phototransducerdevice increases significantly with the value of the input power. In allcases the open circuit voltage (where the responsivity Vs. voltage plotsintersect zero) generated by the subgap phototransducer is substantially13V, despite the fact that the optical excitation energy is below thatof the bandgap energy. Furthermore, the responsivity at higher opticalinput powers, shown at plot 5048 approached the maximum theoreticalresponsivity, plot 5038, for the applied voltages near 0V.

The inventors have also discovered that photon coupling effects orphoton recycling effects can contribute significantly to the improvementof the responsivity in photo transducers based on the VEHSA design asshown in FIG. 26 for example. Such photon coupling and photon recyclingeffects can arise, for example, when one or more of the base segments ofthe VEHSA device generate excess photocurrent with respect to the otherbase segments. Since all base segments are effectively connected inseries, the base segment or base segments that generate excessphotocurrent will effectively become forward biased and may begin toemit photons by radiative recombination in the form ofelectroluminescence. The emitted photons can be reabsorbed by the basesegments which are photocurrent-starved. This reabsorption of theemitted photons will in turn contribute to balance the amount ofphoto-generated current within the various base segments. Such photoncoupling effects or photon recycling effects are more effective athigher excitation intensities when the semiconductor material canapproach a radiative limit. Under these operating conditions, thenon-radiative recombination channels are substantially saturated or arenot competing effectively with the radiative processes. The photonemission from a given base segment will normally be at an energy nearthe edge of the semiconductor bandgap; for GaAs at room temperature thisemission will be around 1.42 eV.

Continuously illuminating a transducer can generate excess heat. Theheat arises from the non-converted optical power. A higher efficiencyphototransducer will therefore generate less heat than a lowerefficiency phototransducer. The generated heat needs to dissipate in thesurrounding environment which can include the package in which thetransducer is mounted, the connections to the package, the mountingapparatus (e.g. a board to which the transducer package is mounted),heatsinks, and the surrounding air, gases, or another cooling mediumincluding liquid cooling. The residual heat can increase the temperatureof the p/n junctions in the phototransducer device. Such an increase intemperature will typically reduce the open circuit voltage and theefficiency of the phototransducer. At least for that reason, it can beadvantageous to operate the phototransducer with pulsed operationinstead of continuous operation. This can be achieved by pulsing thepower of the optical input. The ON duty factor in the pulsedphototransducer operation can be preferably between 100% (always ON) and1/1000000 (ON 0.0001% per time period). The pulse width can bepreferably between 1 fs and 10 s. The pulses width and the repetitionrate can be adjusted to operate the phototransducer in pulse widthmodulation (PWM) mode.

For non-continuous (or non-CW) operation, the area of the device canimpact the response time of the phototransducer. The response time ofthe phototransducer will be affected by the capacitance (C) of thephototransducer and the resistance (R) of the system (the load to whichis electrically connected the transducer). The product of thecapacitance and the resistance can be expressed as an RC constant whichwill impact the response time in pulsed operations. The VEHSA design isadvantageous for such operations because the vertical series-connectedarchitecture effectively combines the p/n junction capacitance inseries. Combining the several capacitance in series reduces theresulting capacitance and therefore increase the response time of thedevice. For applications where relatively fast switching is important,small capacitance and a relatively small phototransducer area can bepreferable. Smaller VEHSA phototransducers can also be advantageous fora given optical input power because the VEHSA device will operate athigher intensities and, as described above, the higher intensitiesenable more efficient photon coupling and photon recycling effects. Forthe efficient operation of VEHSA structures with subgap optical inputs,it can therefore be advantageous to select a smaller active area size,and correspondingly a smaller diameter of the optical input beam, inorder to be effectively operating the phototransducer at higherintensities. The size of the active area of the VEHSA phototransducercan be preferably between 10 micrometers on the side (or in diameter)and 10 cm on the side (or in diameter), and most preferably between 100micrometers and 30 mm.

FIG. 27 shows the relative performance of a same photo transducer as afunction of the wavelength of the optical input. FIG. 27 shows theexpected performance variations of a phototransducer according to thepresent disclosure when the wavelength of the optical input is variedfrom the optimum design value. Plot 5050 and plot 5052 of FIG. 27 showan example, for various optical input wavelengths, of the relativeperformance of a phototransducer with four connecting elements asillustrated and described in FIG. 2. The phototransducer associated withFIG. 27 is designed for an optical input wavelength of 830 nm (i.e. thebase segment thicknesses have been chosen to obtain a current-matchingcondition for a wavelength of 830 nm). The plots 5050 and 5052 thereforedisplay a performance maximum 5054 at an optical input wavelength of 830nm. Plot 5050 corresponds to the performance of the phototransducer whenthe intensity of the optical input is relatively low, for example for aninput power between 0 W and about 1 W as shown in FIG. 25A. At anoptical input wavelength shorter than 830 nm the relative performancedecreases slightly, but it still exceeds 80% of the optimal performanceif the optical input wavelength is changed to ˜800 nm and still exceeds90% if the optical input wavelength is changed to ˜850 nm. However,under the low intensity the subgap performance 5056 is low, or can beclose to zero, because the absorption coefficient of the material of thebase segments is very small.

Plot 5052 corresponds to the performance of the phototransducer when theintensity of the optical input is relatively high, for example for aninput power between 1 W and 5 W as seen in FIG. 25A. In theseconditions, at an optical input wavelength away from the targeted 830 nmthe relative performance decrease slightly, but it still exceeds 95% ofthe optimal performance if the optical input wavelength is changed to˜800 nm and still exceeds 90% if the optical input wavelength is changedto ˜875 nm. The increase in the relative performance for wavelength awayfrom the peak response (part 5058 of plot 5052) is likely due to thephoton recycling or photon coupling effects that are present when higheroptical intensities are used for the input. Furthermore, when theintensity of the optical input is relatively high, the subgapperformance (area 5060 of plot 5052) is now much higher than at the lowintensity portion 5062 of plot 5050. The good subgap performance at highintensities originates a substantial absorption of the material of thebase segments due to the effects disclosed in the other sections above.The plots in FIG. 27 illustrated a rough example of the performancebehavior for different input powers (intensities) and wavelengths, onewith ordinary skill in the art will understand that the details of thebehavior can vary depending on the experimental conditions, but thegeneral increase in performance for the higher input intensities atvarious wavelengths will remain valid by following the guidance of thediscovery hereby disclosed.

FIG. 28 shows the phototransducer conversion efficiency as a function ofthe power of the optical input 100 for a phototransducer with fourconnecting elements as illustrated and described in FIG. 2. Plots 5080and 5082 show increasing efficiencies as a function of optical inputpower. Plot 5080 is for an optical input at a photon energy greater thanthe bandgap energy of the material of the base segments. For example inthese conditions, at 100 mW input, the efficiency can be Eff˜58% and itincreases to Eff˜64.5% at about 2 W of optical input. Plot 5082 is foran optical input at a photon energy below the bandgap energy of thematerial of the base segments. For example in these conditions, at 500mW input, the efficiency can be Eff˜1% but it increases progressively toEff˜35% at about 4 W of optical input. The good subgap performance athigh intensities originates from substantial absorption of the photonsby the material of the base segments due to the effects disclosed in thesections above.

The following relates to a GaAs transducer at a temperature of 298 K(about 25° C.). At this temperature, the bandgap energy is 1.423 eV.With respect to LO phonons, the energy value of an LO phonon in GaAs is36.1 meV, which means that illuminating the GaAs transducer with lighthaving a wavelength of 893.8 nm (1.387 eV) would, with the participationof an LO phonon (36.1 meV), suffice to promote a carrier from thevalence band to the conduction band.

Also at 298 K and but respect to TO phonons, the energy value of a TOphonon in GaAs is 33.2 meV, which means that illuminating the GaAstransducer with light having a wavelength of 891.9 nm (1.390 eV) would,with the participation of an TO phonon (33.2 meV), suffice to promote acarrier from the valence band to the conduction band.

Also at 298 K but with respect to LO and TO phonons, the energy value ofan LO phonon plus the energy of a TO phonon in GaAs is about 69.3 meV,which means that illuminating the GaAs transducer with light having awavelength of 915.7 nm (1.354 eV) would, with the participation of an LOphonon and a TO phonon (69.3 meV), suffice to promote a carrier from thevalence band to the conduction band.

The following relates to a GaAs transducer at a temperature of 313 K(about 25° C.). At this temperature, the bandgap energy is 1.417 eV.With respect to LO phonons, the energy value of an LO phonon in GaAs is36.1 meV, which means that illuminating the GaAs transducer with lighthaving a wavelength of 898.2 nm (1.381 eV) would, with the participationof an LO phonon (36.1 meV), suffice to promote a carrier from thevalence band to the conduction band.

Also at 313 K and but respect to TO phonons, the energy value of a TOphonon in GaAs is 33.2 meV, which means that illuminating the GaAstransducer with light having a wavelength of 896.3 nm (1.383 eV) would,with the participation of an TO phonon (33.2 meV), suffice to promote acarrier from the valence band to the conduction band.

Also at 313 K but with respect to LO and TO phonons, the energy value ofan LO phonon plus the energy of a TO phonon in GaAs is about 69.3 meV,which means that illuminating the GaAs transducer with light having awavelength of 920.3 nm (1.347 eV) would, with the participation of an LOphonon and a TO phonon (69.3 meV), suffice to promote a carrier from thevalence band to the conduction band.

There is also the possibility of illuminating the transducer at phononenergies slightly larger than the bandgap such that the phonon energyminus the various phonon energy equals to the bandgap. For example, at298 K, illuminating a GaAs transducer at a phonon energy of 1423 meV(bandgap energy of GaAs)+69.3 meV (LO+TO phonon)=1489.9 meV (832.2 nm)would/could result in a conversion efficiency better than would be whenilluminating the transducer with light having a substantially largerphoton energy. The improved conversion efficiency at 832.2 nm would beexplained by the generation of an LO and a TO phonon.

Also at 333 K (GaAs bandgap of 1.407 eV) but with respect to a donor oran acceptor energy level as illustrated in FIG. 23, for example for anacceptor energy level (such as C in GaAs for example) the energy valueof the acceptor level can be 40.7 meV, which means that illuminating theGaAs transducer with light having a wavelength of 907.2 nm (1.367 eV)would, with the participation of the acceptor level energy (40.7 meV),suffice to promote a carrier from the acceptor level to the conductionband. And where the impurity energy levels can themselves be populatedor de-populated from the available thermal energy k_(B)T.

With respect to another aspect of the present disclosure, FIG. 29 showsa cutaway view of a prior art optical fiber cable assembly 800 that hasa connector end portion 802. The connector end portion 802 has a ferrule804, a sleeve 806 and a key 808 formed on the sleeve 806. The ferrule804 has on optical fiber 805 located therein. Light coupled to theoptical fiber cable assembly 800 at the remote end (not shown) of theoptical fiber cable assembly 800 will the exit connector end portion802, from the optical fiber 805. The connector end portion 802 also hasa threaded sleeve 810 configured for securing the fiber cable assembly800 to a connector. To connect the connector end portion 802 to aconnector, care must be taken to precisely align the ferrule 804 to thecenter of the connector. An example of such a connector is shown at FIG.30.

In the context of the present disclosure, the optical fiber cableassembly can include a simple optical fiber connected to the connectorend portion and, the connector end portion can be referred to as adelivery end to deliver light propagating through the fiber towards thedelivery end; the optical fiber can be a single mode fiber or amultimode fiber without departing from the scope of the presentdisclosure.

As discussed above, such connectors typically includes alignmentfeatures requiring tight mechanical precision, based on ceramic andmetal elements. Small deviations from tight specifications duringmanufacturing can result in male and female parts that do not fit(connect) well together. Bad connection fittings can be dangerous due topotential disconnections and laser light exposures or due to poorperformances due to higher than normal optical losses. The inventor hasinvented a new connector that allows for easily connecting an opticalfiber cable to a transducer of the present disclosure. The followingdescribes embodiments of such a connector that has higher manufacturingtolerances and allows for more easily connecting optical fiber cables tosuch connectors, which leads to easier field installation or testing.

FIG. 30 shows a cross-sectional view of a prior art flange mountconnector 900, which comprises a threaded sleeve 902 and a ferrulesleeve 904. The ferrule sleeve 904 has a portion 913 that extends awayfrom a wall 915. The ferrule sleeve 904 defines a ferrule cavity 905 inwhich a cylindrical insert 911 is disposed. The cylindrical insert 911can be made of ceramic or metal and has a diameter 907 that closelymatches that of the ferrule 804 (FIG. 29). Such a diameter can be, forexample, 2.5 mm. In most prior art embodiments the cylindrical insert911 is coextensive with the ferrule cavity length. The tolerance of thediameters between the ferrule 804 and the cylindrical insert 911typically has to be of the order of a few thousandths of an inch orless. Similarly, the outside diameter of the portion of the ferrulesleeve 904 that extends away from the wall 915 is typically about 4.5 mmand must have tight tolerances to fit inside diameter of the sleeve 806(FIG. 29). Similarly, the inside diameter of the threaded sleeve 902 istypically 6.18 mm and must have tight tolerances to fit to the outsidediameter of the sleeve 806 (FIG. 29).

The connector 900 defines a device cavity 906 in which a device (e.g., aphotonic device) can be placed and secured through any suitable meanssuch as, for example, an adhesive or solder. The connector 900 isdesigned to allow precise fiber to device alignment (the “device” to belocated in the cavity 906). To connect the optical fiber cable assembly800 (FIG. 29) to the connector 900, a technician is required to grip theconnector end portion 800, align the ferrule 804 with the cylindricalinsert 911, push the connector end portion 800 so the ferrule 804penetrates the cylindrical insert, and then screw the threaded sleeve810 onto the threaded sleeve 902. As will be appreciated by the skilledworker, this operation must be carried out carefully, especially in viewof the tight tolerances mentioned above, in order to avoid scratchingthe face of the ferrule 807 and damaging the optical fiber 805, orpossibly breaking, scratching, or chipping the ceramics of theassemblies from an improperly angled alignment during the connectionprocess. For example, the cylindrical insert 911 and/or the ferrule 804can be prone to breaking when proper care is not taken when connectingthe optical fiber cable assembly 800 to the connector 900.

The phototransducers of the present disclosure are not very sensitive tobeam misalignments or to beam non uniformities and do not require veryprecise alignment with a light source. That is, the phototransducers ofthe present disclosure have a strong tolerance to beam misalignments orto beam non uniformities; this is due to the monolithic verticalarchitecture of the phototransducer's VEHSA design. The verticalarchitecture ensures that each base segments receives substantially thesame photon flux, independently of the beam position on the active area,and independently of the beam profile.

As such, the prior art connectors designed for precise alignment of anoptical fiber to a device, such as the connector 900 of FIG. 30, are notrequired when aligning a connector end portion to a phototransducer ofthe present disclosure. Unfortunately, connectors for low alignmentsensitivity applications appear to be unavailable and only highprecision connectors such as connector 900 can readily be obtained. Itis also commonly found that the tolerances of the prior art commercialconnectors are not always consistent between various vendors. Thesevariations in the tolerance of the commercial male connectors (connectorend portion 802 of FIG. 29) make it difficult to manufacture a universalconnector matching the female connector of the prior art as shown inFIG. 30.

FIG. 31 shows a cross-sectional view of an embodiment of such aconnector 950, which, as the connector 900 of FIG. 30 is a flange mountconnector. The connector 950 has a flange 951, a threaded sleeve 952 anda ferule cavity 955. The connector 950 defines a device cavity 956 inwhich a device (e.g., a photonic device) can be placed and securedthrough any suitable means such as, for example, an adhesive or solder.Light exiting the ferrule (not shown) propagates from the ferrule cavity955, through the aperture 961 and reaches the device cavity 956.

The ferrule cavity 955 any portion that extends away from the wall 915.The diameter 957 of the ferrule cavity 955 of the connector 950 islarger than the diameter 907 of the cylindrical insert 911 of the priorart connector 900. The diameter 957 must be greater than 2.5 mm toaccommodate the 2.5 mm ferrule 804 of FIG. 29. The connector 950 canclearly offer much greater tolerance to accommodate for easymanufacturability and different tolerances from various vendors of maleconnectors 802. For example, the diameter 957 can be between 2.55 mm to6.0 mm, and preferably between 2.6 mm and 3.6 mm.

The connector 950 is a flange mount connector; however, other types ofconnector mounts are also within the scope of the present disclosure.For example, FIG. 32 shows a top front perspective view of an opticalfiber cable assembly 800 facing a board mount connector 960, which has athreaded sleeve 952 and a ferrule cavity 955. The board mount connector960 differs from the flange mount connector 950 in how the connector ismounted to another piece of equipment. As will be understood by theskilled worker, the board mount connector 960 mounts to a board by usingfasteners inserted in holes 962; the flange mount connector 950 mountsto a partition or wall of some sort through fasteners inserted in holes951 (FIG. 31).

FIG. 33 shows a top back perspective view of the connector 960 and theoptical fiber optic cable 800 of FIG. 32. Shown in FIG. 33 is theferrule 804, the sleeve 806, the threaded sleeve 810 and the key 808.Also shown is a notch 809 defined by the threaded sleeve 952. The notchis for receiving the key 808. Further, a transducer 970 is shown securedto the surface mount connector 960.

As will be understood by the skilled worker, neither the notch 809 northe key 808 are required for connecting the optical fiber cable to thesurface mount connector. However, due to the omnipresence of keyedoptical fiber cables, having notched connectors available is important.

FIG. 34 shows the optical fiber cable assembly 800 mounted (connected)to the board mount connector 960.

As will be understood by the skilled worker, the optical fiber cable 800described in the embodiments above is an FC optical fiber cable.However, the connectors of the present disclosure are not limited toconnectors for FC optical fiber cables.

Several other optical fiber cable types exist commercially, including STand SMA optical fiber cables. While the details of the dimensions and ofthe connection mechanisms can be different for ST and SMA connectorsthan for the connector embodiment described above, the principlesexemplified with the above FC connectors are applicable to other typesof connectors. Similarly, while the ferrule 804 of FIG. 29 typically hasan outside diameter of 2.5 mm in outside diameter, male FC connectorswith other ferrule diameters can be readily accommodated with theconnector embodiment discussed above or by modifying the embodiment inquestion within the scope of the proposed design.

FIG. 35 shows an embodiment of a system 1500 in accordance with thepresent disclosure. The system comprises a light source 1502 that isconfigured to generate light that has a pre-determined photon energy.The system 1500 further comprises a transducer 1504 that is configuredto receive light from the light source and to convert optical energy toelectrical energy. The transducer has a plurality of semiconductorlayers electrically connected to each other in series. Eachsemiconductor layer has substantially a same composition that hasassociated thereto a substantially same bandgap energy. The bandgapenergy is greater than the predetermined photon energy.

The light source can include an optical fiber or an optical fiber cablethat has a delivery end to deliver light to the transducer. The systemcan further comprises a connector to which the delivery end is secured.The connector is configured to align the delivery end and the transducerwhen the delivery end is secured to the connector. FIG. 36 shows aflowchart of a method in accordance with the present disclosure. Ataction 1600, a transducer is provided. The transducer has a plurality ofsemiconductor layers electrically connected to each other in series.Each semiconductor layer has substantially a same composition that hasassociated thereto a substantially same bandgap energy. At action 1602,the transducer is illuminated with light that has associated thereto aphoton energy that is less than the bandgap energy.

Another aspect of the present disclosure provides a monolithicsemiconductor connecting element structure or connecting layer(s) toelectrically connect at least two different parts of an optoelectronicdevice grown by epitaxy. The connecting element, in some embodiments,provides high transparency for photons impinging the input active areaof the device that need to be transmitted through the connecting elementwith no significant optical absorption. The low electrical resistance isobtained by pseudomorphically growing a low resistance tunnel junctionvia use of an epitaxy processes. The low resistance properties of thetunnel junction are obtained by achieving p++ and n++ layers with highdoping concentrations, which have an average lattice constant that doesnot deviate significantly from the underlying layer upon which it isgrown (also known as pseudomorphic growth). For example, in devices thatare lattice-matched to the GaAs lattice constant (a˜0.56575 nm), thehigh p-type doping concentration (sometimes also called p++ layer) of atunnel junction can often be achieved by doping that semiconductor layerwith carbon (C) to a level between 1E18 cm-3 to 5E21 cm-3, andpreferably to a level between 5E19 cm-3 to 5E20 cm-3. An example of sucha device 3700 is depicted in FIG. 37.

Similarly, in devices that are lattice-matched to the GaAs latticeconstant (a˜0.56575 nm), the high n-type doping concentration (sometimesalso called n++ layer) of a tunnel junction can often be achieved bydoping that semiconductor layer with carbon (Te) to a level between 1E18cm-3 to 5E21 cm-3, and preferably to a level between 5E19 cm-3 to 5E20cm-3. Other dopants can be used, but the epitaxy process must allowachieving the targeted doping levels. For example, Se, S, or O could beused instead of Te. Depending on the energy gap of the active layer thatare on the side opposite to the optical input active area, the abovesemiconductor layers comprised in the connecting element can be chosento be latticed matched GaAs or a quasi-lattice-matched AlxGa(1−x)Asalloy, with x for example between 0% and 100%, and most preferablybetween 0% and 40%.

There exists epitaxy processes that can be used to readily achieve thedesired n++ and p++ doping profile, as mentioned above, for such GaAs orAlxGa(1−x)As. It will be apparent for one skilled in the art that such adesign will result in low-resistance lattice-matched or pseudomorphictunnel junctions, which can be used as connecting elements that featurethe desired low-resistivity and high-transparency properties on GaAslayers. Typically the low resistivity desired will results in a drop involtage preferably lower than 100 mV and most preferably lower than 10mV, depending on the current passed through the optoelectronic devices.For concentrated photovoltaic devices or for laser power converterdevices, the current passing through such connecting element can be ofthe order of several amperes (for example typically smaller than 10 A),and the device area is preferably less than 10 mm×10 mm but can besometimes a few cm x a few cm depending on the application, and mostpreferably less than 6 mm×6 mm. The is no limit on how small the devicearea can be other than practical geometrical limits, but preferably thedevice area is larger than 0.1 mm×0.1 mm, and most preferably largerthan 1.0 mm×1.0 mm. The device area can be square, rectangular, or haveany arbitrary geometry depending on the application, and theparticularity of the geometry of the device area is not affecting thegenerality of the present disclosure.

The thicknesses of the p++ and the n++ layers described above aretypically between 1000 nm and 0.1 nm, preferably between 100 nm and 1nm, and most preferably between 90 nm and 10 nm. The thickness of thep++ layer can be designed to be about the same as that of the n++ layer,or similar results can be obtained in the case for which the thicknessof the p++ layer can be different than that of the n++ layer. Thedetermination of the optimal thickness for these layers can typicallytake into consideration carrier depletion effects at interfaces,diffusion effects in the growth or post growth processes, the ramp upand ramp down time of the dopant profile during the epitaxial growthwith respect to the growth rate of the alloys used in the n++ and p++layers, the thickness of the depletion regions in the formation of theselayers, the Coulomb effects for the carriers in the formation of theselayers, the effects of the portion of the activated dopants, the surfacesegregation effects of the dopants or of the alloys during the epitaxyprocess, surface reconstruction effects, surface morphology effects,dopant incorporation efficiency effects, lattice contraction orexpansion effects caused by heavy dopant concentrations, quantum well orlow-dimensionality tunneling effects, space-charged effects, or anyother effects affecting the active carrier concentrations, the effectiveelectric fields participating in the quantum tunneling phenomena, theresidual optical absorption in these layers, or the doping profiles.

For example, FIG. 37 depicts prior art connecting elements based on suchtunnel junctions grown with the GaAs lattice on a GaAs layer 3776. Inparticular, FIG. 37 shows two such connecting elements 3782 and 3790.Other connecting elements are not shown but can be incorporated withinthe other layers 3786, which can comprise other parts of the structure3700 and other such connecting elements for structure comprising amultitude of repeated parts. As will be understood by one with ordinaryskill in the art, while FIG. 37 depicts prior art connecting elements,such prior art connecting elements can in some instances be used tocreate novel embodiments, but in other instances, the prior artconnecting elements might be too limiting to construct other novelembodiments as described in more details further below.

In FIG. 37, the connecting element 3782 connects a first part of thestructure 3780 to a second part of the structure 3784. Similarly, theconnecting element 3790 connects a N−1 part of the structure 3788 to a Npart of the structure 3792. The incoming photons 3704 impinge on theoptical input layers 3708 and are transmitted to the first part of thestructure 3780, which can absorb some of the photons 3704. The photonspectrum transmitted through structure 3780 is then passed through theconnecting element 3782. The connecting element 3782 is said to havehigh optical transparency if it does not significantly absorb thespectral content of the optical input 3704 that was transmitted throughthe first part of the structure 3780 and impinge on the connectingelement 3782. In such case, substantially all the light impinging fromone side of the connecting element 3782 is transmitted to the other sideto the second part of the structure 3784. Similarly, the connectingelement 3782 can be said to have a low resistivity (or equivalently ahigh-conductivity), if the electrical resistance across the connectingelement 3782 is smaller than 100 millohms, and preferably smaller than10 milliohm for example. The exact value of the resistance that willdefine a low resistivity will depend on the application of the deviceand the area of the device used. In such a device, the p++ layers 3724,3756 of the connecting elements 3782, 3790, respectively, can be forexample a single layer of GaAs(C) and the n++ layers 3728, 3760 of theconnecting elements 3782, 3790, respectively, can be a single layer ofGaAs(Te) for example. The doping level of the n++ and p++ layers 3724,3728, 3756, 3760 can be chosen with doping values as mentioned above.Furthermore, the GaAs 3776 is lattice-matched to the GaAs latticeconstant so the layers can readily be grown by epitaxy using commercialreactors for example.

The epitaxy can be carried out through any suitable means ofsemiconductor growth such as: metal organic chemical vapor deposition(MOCVD), chemical beam epitaxy (CBE), molecular beam epitaxy (MBE),solid phase epitaxy (SPE), hydride vapor phase epitaxy or by othersimilar hybrid systems or combinations thereof. The growth parameterscan be optimized for the various layers of the embodiments, for exampleto maximize the device performance or its manufacturability. The growthparameters and growth conditions that can be optimized include, forexample, the growth temperature, the pressure of the various gases usedto grow the layers, the ratio of those pressures (for example the III/Vratio when growing III-V semiconductor layers), the alloy composition,the residual strain, the growth rate, the doping or co-doping of thevarious layers, the use of surfactant gases, the use of annealingcycles, etc.

As shown in FIG. 37, the structure 3700 may include the optical inputlayer(s) 3708 built on top of a first part of the structure 3780, whichis built on top of a first connecting element 3782, which is built ontop of a second part of the structure 3784, . . . , which is built ontop of an N−1 part of the structure 3788, which is built on top ofanother connecting element 3790, which is built on top of an Nth part ofthe structure 3792, which is built on top of the GaAs layer 3776. Itshould be appreciated that each of the parts and connecting elements mayinclude one, two, three, or more layers. For example, the first part ofthe structure 3780 is shown to include an n-type side 3712, a first partwith bandgap Eg1 3716, and a p-type side 3720. Similarly, the secondpart of the structure 3784 is shown to include an n-type side 3732, asecond part with bandgap Eg1 3736, and a p-type side 3740. Furthermore,the N−1 part of the structure 3788 is shown to include an n-type side3744, an N−1 part with bandgap Eg1 3748, and a p-type side 3752. Lastly,the N part of the structure 3792 is shown to include an n-type side3764, an N part with bandgap Eg1 3768, and a p-type side 3772. As can beappreciated, each part of the structure includes at least a p-type layerand an n-type layer. Each p-type layer and n-type layer may have abuilt-in photovoltage that is substantially similar to the built-inphotovoltage of all other parts of the structure.

The stack 3700 is also shown to have a plurality of connecting elementsor connecting layers 3782, 3790 that are sandwiched between two adjacentp-n stacks, thereby providing electrical communication between theadjacent stacks. Because there exists applications that requiredifferent optical properties than the ones described in FIG. 37,variations of the structure 3700 may be useful. For example, for someapplications it is desirable to have a lower bandgap than the bandgap ofGaAs as exemplified above with the structure depicted in FIG. 37. Forsuch applications, it can be desirable to construct a device 3800 on thebasis of an InyGa(1−y)As layer 3804, as shown in FIG. 38. The device3800 may be similar to device 3700 in that a stack of p-n layers andconnecting elements are built on top of a substrate. It should beappreciated, however, that the difference between layer 3804 and 3704may result in different materials, dopants, and/or doping concentrationsused in some or all of the p-n layers and/or connecting elements. Whilesimilar reference numbers as used in FIG. 37 will be used to describesome of these p-n layers and connecting elements of FIG. 38, it shouldbe appreciated that the p-n layers and/or connecting elements may bedifferent in substance.

The InyGa(1−y)As layer 3804 can be a metamorphic layer or a virtualsubstrate, for example. In that case, however, one skilled in the artwould recognize that the lattice constant of the base layer is larger.For example, for y=12%, the lattice constant of layer 3804 is nowapproximately 0.57016 nm, and the In0.12Ga0.88As will have a bandgap of1.247 eV at 20° C. which would correspond to a threshold absorptionbandage wavelength of 994 nm. Such an embodiment is useful for infraredapplications, for example.

The In0.12Ga0.88As semiconductor alloy can be used for example in laserpower converting devices for an optical input 3704 based on a high-power976 nm laser source. Such lasers are readily available commercially withhigh powers, good reliability, and are also cost-efficient optical powersources. Because the base layer 3804 now has a different latticeconstant, it is also desirable to change the design of the connectingelements 3782, 3790 to maintain the pseudomorphic epitaxial growth.Indeed, if a connecting element is not lattice-matched orpseudomorphically grown to its underlying layers, then defects areexpected to form after a critical thickness and the defected materialcan be expected to have poor electrical properties. The value of thecritical thickness that can be attained before the formation of defectsis dependent on the lattice-mismatch with the adjacent layers. A highdensity of defects in the connecting element 3782, 3790 is expected totrap the carriers and reduce the peak tunneling current. The peaktunneling current is the characteristic current of a tunneling diodebefore its current-voltage behavior enters into a negative differentresistance (NDR) regime, whereby the current through the device startsto decrease with increasing voltage. For a tunnel junction to be highlyconductive, it is desirable to operate the tunnel junction at a currentsignificantly lower than its peak tunneling current. It is, therefore,desirable to realize tunnel junctions with high peak tunneling currents.In practice, this translates into the constraints of maintainingpseudomorphic layers for the tunnel junction design, and achieving highdoping levels in these layers.

FIG. 39 illustrates an embodiment for the above example based on anIn0.12Ga0.88As semiconductor alloy for the base layer 3804. A highbandgap InGaP 3732 will be lattice-matched to the In0.12Ga0.88As basedlayer 3804 for an indium composition of 60%. It will have a bandgap of1.768 eV and can be doped n-type using Si, for example. Similarly, thep-type cladding layer 3720 can be designed with In60% Ga40% P by dopingwith Zn for example. For the p++ layer 3724 and the n++ layer 3728, adesign based on an In12% AlzGa(0.88−z)As alloy will be lattice-matchedto the In0.12Ga0.88As based layer 3804. The aluminum composition oflayer 3724 and 3728 can be set between 0% and 100%, and preferablybetween 5% and 35% to maintain a good transparency of the connectingelement 3782 for the impinging photons. Although FIG. 39 depicts detailsof a connecting element 3782, it should be appreciated that connectingelement 3790 and other connecting elements may be similarly oridentically constructed.

However, while the embodiment of FIG. 39 satisfies the lattice-matchingconstraint, it also introduces a significant alloy fraction of indiuminto the n++ and p++ tunnel junction layers 3724, 3728. The indiumcomposition of these layers can make it difficult to reach the desireddoping level. For example, in MOCVD, it is difficult to reach highp-type doping with C when In is present in the layer. Similarly, thepresence of indium can make it difficult or impossible to reach therequired n++ doping level by using Te or Se as a dopant. Other dopantspecies or growth conditions can be attempted to reach the requireddoping levels, but typically yield unsatisfactory results and/or growthconditions that are not favorable in view of manufacturing or costconsiderations.

The following discloses a solution that will overcome the above problemsof obtaining the required n++ and p++ doping level while maintaining therequired pseudomorphic structural properties for the infraredoptoelectronic applications requiring a bandgap lower than GaAs, such asInxGa1−xAs, including In0.12Ga0.88As.

More specifically, FIG. 40 shows an embodiment whereby a series of layerpairs are used to construct the connecting element 3782 and/or 3790 thatare illustrated in FIG. 38. In some embodiments, the layers depicted inconnection with FIG. 40 and/or FIG. 38 may correspond to a stack oflayers operating as a power over fiber or a laser power converterdevice.

Layers 4008, 4020, 4028, 4040 are designed based on an AlzGa1−zAs with athickness t1. Layers 4012, 4024, 4032, 4044 are designed based on thebinary semiconductor InAs with a thickness t2. The Al composition z isthen chosen to obtain the desired optical transparency while the valuesof t1 and t2 are selected to maintain pseudomorphic epitaxial layers ofhigh quality. An advantage of this embodiment is that the AlzGa1−zAslayers are designed to not comprise any indium. They can therefore bereadily doped n++ and p++ with the desired doping level using standardepitaxy processes. For example, in MOCVD for example, the Te is known toincorporate well to achieve high doping values in AlGaAs. Similarly, inMOCVD for example, C is known to incorporate well to achieve high dopingvalues in AlGaAs. We can therefore refer to layers 4008, 4020, 4028,4040 as doping tunnel junction layers, or layers made of highly dopablematerial; and we can refer to layers 4012, 4024, 4032, 4044 as adjustortunnel junction layer. Therefore, the present disclosure resolves thedoping constraint of for connecting element for the infraredoptoelectronic applications requiring a bandgap lower than GaAs, such asInxGa1−xAs, including In0.12Ga0.88As. It will then become clear from oneskilled in the art that many other useful configurations will be usefulfor other applications, for example, for an optical input around 910 nm,a similar embodiment can be constructed with a In0.08Ga0.92As baselayer, and by making the related adjustments in the thicknesses t1 andt2 of the above embodiment.

For example, to further describe the example of FIG. 40, whereas thebase layer is In0.12Ga0.88As, the values of t1=2.2 nm and t2=0.3 nm willresult in an average lattice constant equal to In0.12Ga0.88As (herea˜0.57016 nm at 20° C.). The values of t1 and t2 above have beenselected such that the individual layers are below the criticalthickness at which dislocations will start to form. Furthermore, sincethe t1 and t2 are thin in view of the electron and hole wave functionsin these material, the wavefunction coupling will form an effectivecoherent or partially-coherent state that will substantially behave as amonolithic alloy. A skilled artisan will recognize that other values oft1, t2, and alloy compositions will similarly result effectivepseudomorphic alloys with the desired properties. The mainconsiderations in the selection of t1 and t2 are that each individuallayer thickness remains below the critical thickness, and that theweighted average of the respective lattice constants yields an averagelattice constant matched to the base layer (here In0.12Ga0.88As in thisexample). For example, the critical thickness for InAs is known to beabout 1.5 monolayer. Since an InAs thickness of t2=0.3 nm wouldcorrespond roughly to 1 monolayer, the growth will proceed in a planarlayer-by-layer growth mode, and the resulting layer sequence willproceed pseudomorphically.

The effective bandgap of the connecting element 3782, 3790 can beevaluated for various values of Al composition z. The effective bandgapestimate can be approximated by using the weighted average of theindividual bandgaps. This approximation can be refined by calculatingthe quantized energy levels in the selected design. For the example inFIG. 40, with t1=2.2 nm and t2=0.3 nm, and z=15%, the weighted averageestimate predicts an effective bandgap of 1.462 eV, whereas the quantumheterostructure calculation predicts an effective bandgap of 1.446 eV(16 meV lower). For both estimates, the optical input beam impinging ata wavelength of 976 nm will not be absorbed within the connectingelement 3782, 3790.

FIG. 41 summarizes the effective bandgap evaluation (curve 4110) as afunction of the aluminum composition of the AlzGa(1−z)As alloy for anembodiment based on t1=2.2 nm and t2=0.3 nm. For all values of z, theconnecting element layers are pseudomorphic to In12% Ga88% As. Thisembodiment is therefore suitable for example for an input optical powerimpinging at 976 nm. Curve 4120 depicts the optical input beam energy.By selecting an appropriate energy difference (curve 4130) between theoptical input beam (curve 4120) and the effective bandgap of theresulting connecting element (curve 4110), the high optical transparencyof the connecting element can be insured. As mentioned above, thedesired n++ and p++ doping levels are achieved by doping the AlGaAslayers 4008, 4020, 4028, 4040, etc with Te and with C respectively forexample. The desired high peak current of the resulting tunnel junctionis therefore achieved by doping the AlGaAs layers 4008, 4020, etc withcarbon (C) to a level between 1E18 cm-3 to 5E21 cm-3, and preferably toa level between 5E19 cm-3 to 5E20 cm-3; and by doping the AlGaAs layers4028, 4040, etc with carbon (Te) to a level between 1E18 cm-3 to 5E21cm-3, and preferably to a level between 5E19 cm-3 to 5E20 cm-3. As shownin FIG. 40, repeating AlGaAs/InAs pairs 4020, 4036 may be providedbetween p++ pairs and n++ pairs.

As shown in FIG. 42, in addition to the high optical transparencyrequirements mentioned in the description above, the standard tunneljunctions 4204 have two key attributes related to achieving theirrequirement of low resistivity: 1) they should have high doping levelsto be highly conductive and 2) they should match the lattice spacing ofthe material A to connect to prevent dislocations and defects. Tunneljunctions 4204 are basic building blocks that can be used in somesemiconductor devices namely, stacked lasers, solar cells andphoto-transducers or photo-converters. Embodiments of the presentdisclosure contemplate that any type of tunnel junction depicted anddescribed herein can be used in a transducer, stacked laser (or similarlight-emitting device), solar cells, photo-converters, or any othersemiconductor device.

However, this is restricted to materials (e.g., material A) with alattice constant identical or substantially identical to that of thestandard tunnel junctions. Materials with different lattice constantsgenerally cannot be connected without dislocations, defects and otherfaults that prevent proper working of the connection. As a more specificexample, for materials with a different lattice spacing, the standardtunnel junctions may not have a lattice spacing with the proper match.This will lead to dislocations and defects at the interfaces. Thedifficult problem is that materials with different lattice spacing aredesirable for lasers or photo-transducers operating in specificwavelength ranges.

In accordance with at least some embodiments of the present disclosure,and as shown in FIGS. 43 and 44, an improved tunnel junction 4304 isprovided that includes standard tunnel junction materials TJ1 and TJ2periodically interspaced by an additional material, which may bereferred to herein as a lattice adjuster or (LA) 4404, that forces thetunnel junction 4304 to adopt the lattice constant of a material B to beconnected. As mentioned in the above description, the lattice adjusterlayer 4404 can therefore also be refer to as an adjustor tunnel junctionlayer. This approach gives a good epitaxial connection withoutdislocations and defects. As shown in FIG. 44, the standard tunneljunction materials TJ1 and TJ2 are either stretched or compressed tomatch the other material lattice spacing by the presence of the latticeadjuster material 4404, which has the opposite stress/strain. Forexample, two different materials TJ1 and TJ2 for a tunnel junction 4304would be stretched and the lattice adjuster material 4404 would becompressed to counteract the stretching of the tunnel junction materialsTJ1 and TJ2.

In epitaxial semiconductor growth, stretching/compression occurs when amaterial is grown with a “natural” (bulk) lattice constant smaller thanthat of the substrate. As the material grows it is forced to adopt thelarger lattice constant and it is stretched in the plane of thesubstrate. It should be noted that it is also compressed in this case inthe direction perpendicular to the substrate. The resulting effect isthat the wafer or substrate will be bowed upwards. There is a limit tothis elastic behavior and too much difference in the lattice constantslead to dislocations, breaking defects, or the like. For compression theopposite behavior is revealed, i.e. trying to grow a material with a“natural” (bulk) lattice constant larger than that of the substrate. Inthis situation, bowing will occur in the opposite direction. Embodimentsof the present disclosure overcome these issues by presenting a tunneljunction 4304 with one or more lattice adjuster materials 4404.

It should also be appreciated that the wavelength of interest determinesthe materials to select. For example, if it is desired to convert lightfrom a laser at a wavelength of 975 nm, a good candidate for theabsorber material is the ternary compound In(12%)Ga(88%)As with abandgap with an energy only slightly smaller to the incoming photonswith a wavelength of 975 nm. This ternary material however will not growwell on known epitaxial wafer substrates like GaAs, Si, InP, Ge, orothers. It can, however, grow well on so-called “metamorphic” layers orvirtual substrates. Once the substrate and p-n junctions material arechosen, the materials for the tunnel junction 4304 can be selected. Asstated earlier, the choice of tunnel junction material is limitedbecause of the high doping and the optical transparency requirements.Accordingly, a material like AlGaAs—GaAs can be selected, which is notlattice matched to the ternary/metamorphic substrate. This materialselection may otherwise be subject to dislocations and/or breaking ifnot appropriately addressed. Embodiments of the present disclosureaddress these potential problems by introducing the lattice adjustor4404 layers between tunnel junction materials TJ1, TJ2. The latticeadjustor materials 4404 provide the opposite stress within the tunneljunction 4304 to manage the stress build-up.

Similar considerations would apply, for example, when implementinghigh-performance tunnel junctions on other material for otherapplications. For example on InP, the p++ dopable material could can bep++AlGaAs(C) and the n++ dopable material could can be n++AlGaAs(Te),whereby the lattice adjustor layers 4404 could again be InAs (orequivalently (InGaAs with a high In fraction), but now thicker toproperly take into account the different lattice constant requirements.As also mentioned above, other examples can be envisioned, such asIn(8%)Ga(92%)As which would be useful for applications in the 900-910 nmrange. It should also be appreciated that the tunnel junction 4304 andlattice adjustor materials 4404 provided therein can be useful fordifferent types of devices such as lasers or emitters instead of powerconverters or transducers.

At least one embodiment is disclosed and variations, combinations,and/or modifications of the embodiment(s) and/or features of theembodiment(s) made by a person having ordinary skill in the art arewithin the scope of the disclosure. Alternative embodiments that resultfrom combining, integrating, and/or omitting features of theembodiment(s) are also within the scope of the disclosure. Wherenumerical ranges or limitations are expressly stated, such expressranges or limitations may be understood to include iterative ranges orlimitations of like magnitude falling within the expressly stated rangesor limitations (e.g., from about 1 to about 10 includes, 2, 3, 4, etc.;greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example,whenever a numerical range with a lower limit and an upper limit isdisclosed, any number falling within the range is specificallydisclosed. Additionally, the use of the term “substantially” means+/−10%of a reference value, unless otherwise stated. As an example, thephrase: “voltage A is substantially the same as voltage B” means thatvoltage B is within +/−10% of voltage B.

In the preceding description, for purposes of explanation, numerousdetails are set forth in order to provide a thorough understanding ofthe embodiments. However, it will be apparent to one skilled in the artthat these specific details are not required. In other instances,well-known electrical structures and circuits are shown in block diagramform in order not to obscure the understanding. For example, specificdetails are not provided as to whether the embodiments described hereinare implemented as a software routine, hardware circuit, firmware, or acombination thereof.

The above-described embodiments are intended to be examples only.Alterations, modifications and variations can be effected to theparticular embodiments by those of skill in the art without departingfrom the scope, which is defined solely by the claims appended hereto.

What is claimed is:
 1. A transducer configured to convert optical energyto electrical energy, the transducer comprising: a plurality of p-nstacks, wherein each p-n stack comprises at least a p-layer and ann-layer, and formed therein a built-in photovoltage between the p-layerand the n-layer, wherein all of the p-layers in the plurality of p-nstacks comprise substantially the same p-type material in thesubstantially the same composition, and wherein all of the n-layers inthe plurality of p-n stacks comprise substantially the same n-typematerial in substantially the same composition such that each p-n stackin the plurality of p-n stacks has a substantially similar built-inphotovoltage; a plurality of connecting layers, each connecting layer inthe plurality of connecting layers being sandwiched between two adjacentp-n stacks for electrically connecting the two adjacent p-n stacks,wherein each of the p-n stacks in the plurality of p-n stacks isarranged such that the built-in photovoltage of each p-n stackadditively contributes to an overall electric potential of thetransducer; and wherein each p-n stack in the plurality of p-n stackshas a lattice spacing layer and wherein each connecting layer in theplurality of connecting layers comprises a doping tunnel junction layerhaving a lattice spacing type of either a stretched type or a compressedtype relative to the lattice spacing layer of the p-n stack as well asan adjustor tunnel junction layer having a lattice spacing type which isopposite the doping tunnel junction layer.
 2. The transducer of claim 1,wherein each connecting layer in the plurality of connecting layerscomprises a group of layers with material or composition different thanan adjacent p-layer and different than an adjacent n-layer.
 3. Thetransducer of claim 1, wherein each p-n stack in the plurality of p-nstacks further comprises a least a bandgap energy layer sandwichedbetween the p-layer and the n-layer thereof.
 4. The transducer of claim1, further comprising a passivation layer facing a light source, whereinone of the p-layer and n-layer of each p-n stack comprises a photoncatching region with a thickness that increases away from thepassivation layer to the photon catching region such that asubstantially similar amount of photons are captured in each photoncatching region.
 5. The transducer of claim 1, wherein each connectinglayer further comprises a plurality of additional doping tunnel junctionlayers and a plurality of additional adjustor tunnel junction layers. 6.The transducer of claim 5, wherein: the doping tunnel junction layer andthe plurality of additional doping tunnel junction layers have a firsttunnel junction thickness; the adjustor tunnel junction layer and theplurality of additional adjustor tunnel junction layers have a secondtunnel junction thickness; and the first tunnel junction thickness andthe second tunnel junction thickness are selected such that theconnecting layer adopts the lattice spacing of the p-n stack.
 7. Thetransducer of claim 1, wherein the doping tunnel junction layer has ap-type doping tunnel junction layer and an n-type doping tunnel junctionlayer.
 8. The transducer of claim 1, wherein the adjustor tunneljunction layer has a p-type adjustor tunnel junction layer and an n-typeadjustor tunnel junction layer.
 9. An optoelectronic device configuredto convert electrical energy to optical energy, comprising: a pluralityof p-n stacks, wherein each p-n stack comprises at least a p-layer andan n-layer, wherein all of the p-layers in the plurality of p-n stackscomprise substantially the same p-type material in the substantially thesame composition, and wherein all of the n-layers in the plurality ofp-n stacks comprise substantially the same n-type material insubstantially the same composition; and a plurality of connectinglayers, each connecting layer in the plurality of connecting layersbeing sandwiched between two adjacent p-n stacks for electricallyconnecting the two adjacent p-n stacks, wherein each p-n stack in theplurality of p-n stacks has a lattice spacing layer and wherein eachconnecting layer in the plurality of connecting layers comprises adoping tunnel junction layer having a lattice spacing type of either astretched type or a compressed type relative to the lattice spacinglayer of the p-n stack as well as an adjustor tunnel junction layerhaving a lattice spacing type which is opposite the doping tunneljunction layer.
 10. The optoelectronic device of claim 9, wherein eachof the p-n stacks in the plurality of p-n stacks is arranged such thatthe built-in photovoltage of each p-n stack additively contributes to anoverall electric potential of the optoelectronic device.
 11. Theoptoelectronic device of claim 9, wherein each connecting layer in theplurality of connecting layers comprises a group of layers with materialor composition different than an adjacent p-layer and different than anadjacent n-layer.
 12. The optoelectronic device of claim 9, wherein eachp-n stack in the plurality of p-n stacks further comprises a least abandgap energy layer sandwiched between the p-layer and the n-layerthereof.
 13. The optoelectronic device of claim 9, wherein: eachconnecting layer further comprises a plurality of additional dopingtunnel junction layers and a plurality of additional adjustor tunneljunction layers; the doping tunnel junction layer and the plurality ofadditional doping tunnel junction layers have a first tunnel junctionthickness; the adjustor tunnel junction layer and the plurality ofadditional adjustor tunnel junction layers have a second tunnel junctionthickness; and the first tunnel junction thickness and the second tunneljunction thickness are selected such that the connecting layer adoptsthe lattice spacing of the p-n stack.
 14. The optoelectronic device ofclaim 9, wherein the doping tunnel junction layer has a p-type dopingtunnel junction layer and an n-type doping tunnel junction layer. 15.The optoelectronic device of claim 9, wherein the adjustor tunneljunction layer has a p-type adjustor tunnel junction layer and an n-typeadjustor tunnel junction layer.
 16. The optoelectronic device of claim9, wherein the optical energy emitted by the optoelectronic devicecomprises a collimated beam of light having a predetermined wavelength.17. A semiconductor, comprising: a first p-type layer having a p-typematerial of a first composition; a second p-type layer having the p-typematerial of the first composition; a first n-type layer having an n-typematerial of a second composition; a second n-type layer having then-type material of the second composition; a first connecting layersandwiched between the first p-type layer and the first n-type layer,wherein the first connecting layer electrically connects the firstp-type layer with the first n-type layer, wherein the first connectinglayer comprises a first tunnel junction that includes at least twotunnel junction layers separated by a first lattice adjustor layer thatat least partially counteracts a stress or strain induced by the atleast two tunnel junction layers of the first connecting layer; and asecond connecting layer sandwiched between the second p-type layer andthe second n-type layer, wherein the second connecting layerelectrically connects the second p-type layer with the second n-typelayer, wherein the second connecting layer comprises a second tunneljunction that includes at least two tunnel junction layers separated bya second lattice adjustor layer that at least partially counteracts astress or strain induced by the at least two tunnel junction layers ofthe second connecting layer.
 18. The semiconductor of claim 17, whereinthe first p-type layer and the first n-type layer form a first p-nstack, wherein the second p-type layer and the second n-type layer forma second p-n stack, and wherein the wherein the first p-n stack andsecond p-n stack are arranged such that the built-in photovoltage ofeach p-n stack additively contributes to an overall electric potentialof the semiconductor.
 19. The semiconductor of claim 18, wherein thefirst p-n stack and the second p-n stack bother have a lattice spacinglayer and wherein the first connecting layer and the second connectinglayer both comprise a doping tunnel junction layer having a latticespacing type of either a stretched type or a compressed type relative tothe lattice spacing layer of the p-n stack as well as an adjustor tunneljunction layer having a lattice spacing type which is opposite thedoping tunnel junction layer.